A detailed expression map of the PIN1 auxin transporter in Arabidopsis thaliana root.

BACKGROUND: Theauxin efflux carrier PIN1 is a key mediator of polar auxin transport in developing plant tissues. This is why factors that are supposed to be involved in auxin distribution are frequently tested in the regulation of PIN1 expression. As a result, diverse aspects of PIN1 expression are dispersed across dozens of papers entirely devoted to other specific topics related to the auxin pathway. Integration of these puzzle pieces about PIN1 expression revealed that, along with a recurring pattern, some features of PIN1 expression varied from article to article. To determine if this uncertainty is related to the specific foci of articles or has a basis in the variability of PIN1 gene activity, we performed a comprehensive 3D analysis of PIN1 expression patterns in Arabidopsis thaliana roots. RESULTS: We provide here a detailed map of PIN1 expression in the primary root, in the lateral root primordia and at the root-shoot junction. The variability in PIN1 expression pattern observed in individual roots may occur due to differences in auxin distribution between plants. To simulate this effect, we analysed PIN1 expression in the roots from wild type seedlings treated with different IAA concentrations and pin mutants. Most changes in PIN1 expression after exogenous IAA treatment and in pin mutants were also recorded in wild type but with lower frequency and intensity. Comparative studies of exogenous auxin effects on PIN1pro:GUS and PIN1pro:PIN1-GFP plants indicated that a positive auxin effect is explicit at the level of PIN1 promoter activity, whereas the inhibitory effect relates to post-transcriptional regulation. CONCLUSIONS: Our results suggest that the PIN1 expression pattern in the root meristem accurately reflects changes in auxin content. This explains the variability of PIN1 expression in the individual roots and makes PIN1 a good marker for studying root meristem activity.

Identification and characterization of genes associated with the induction of embryogenic competence in leaf-protoplast-derived alfalfa cells.

Alfalfa leaf protoplast-derived cells can develop into somatic embryos depending on the concentration of 2,4-dichlorophenoxyacetic acid (2,4-D) in the initial culture medium. In order to reveal gene expression changes during the establishment of embryogenic competence, we compared the cell types developed in the presence of 1 and 10 microM 2,4-D, respectively, at the time of their first cell divisions (fourth day of culture) using a PCR-based cDNA subtraction approach. Although the subtraction efficiency was relatively low, applying an additional differential screening step allowed the identification of 38 10 microM 2,4-D up-regulated transcripts. The corresponding genes/proteins were annotated and representatives of various functional groups were selected for more detailed gene expression analysis. Real-time quantitative PCR (RT-QPCR) analysis was used to determine relative expression of the selected genes in 2,4-D-treated leaves as well as during the whole process of somatic embryogenesis. Gene expression patterns confirmed 2,4-D inducibility for all but one of the 11 investigated genes as well as for the positive control leafy cotyledon1 (MsLEC1) gene. The characterized genes exhibited differential expression patterns during the early induction phase and the late embryo differentiation phase of somatic embryogenesis. Genes coding for a GST-transferase, a PR10 pathogenesis-related protein, a cell division-related ribosomal (S3a) protein, an ARF-type small GTPase and the nucleosome assembly factor family SET protein exhibited higher relative expression not only during the induction of somatic embryogenesis but at the time of somatic embryo differentiation as well. This may indicate that the expression of these genes is associated with developmental transitions (differentiation as well as de-differentiation) during the process of somatic embryogenesis.

[Site-specific expression of single-stranded antibody fragments in Nicotiana tabacum].

Antibody expression and immunomodulation are modern molecular techniques to produce pharmaceuticals and to interfere with cellular metabolism or pathogen infectivity in plants. Nonetheless, there is still no generally applicable strategy to express correctly folded active antibodies or antibody fragments in different cell compartments. To facilitate expression, single-chain antibody fragments (scFvs) were made of mouse monoclonal antibodies, J2 and P6 that specifically recognize double-stranded RNA (dsRNA). Stabilizing double-stranded replication intermediates could modulate the biological activity of dsRNAs in plants, especially to influence virus replication. Along with cytoplasmic expression, scFvs were anchored to the plasma membrane; targeted to the apoplast for secretion and made ER-resident. Expression levels were analysed and transgenic plants were evaluated for resistance or tolerance to potato virus Y infection. We have established strategies for expression of correctly assembled antibodies or antibody fragments in different plant cell compartments.

Motion information is spatially localized in a visual working-memory task.

We asked if the information about stimulus motion used in a visual working-memory task is localized in space. Monkeys compared the directions of two moving random-dot stimuli, sample and test, separated by a temporal delay and reported whether the stimuli moved in the same or in different directions. By presenting the two comparison stimuli in separate locations in the visual field, we determined whether information about stimulus direction was spatially localized during the storage and retrieval/comparison components of the task. Two psychophysical measures of direction discrimination provided nearly identical estimates of the critical spatial separation between sample and test stimuli that lead to a loss in threshold. Direction range thresholds measured with dot stimuli consisting of a range of local directional vectors were affected by spatial separation when a random-motion mask was introduced during the delay into the location of the upcoming test. The selective masking at the test location suggests that the information about the remembered direction was localized and available at that location. Direction difference thresholds, measured with coherently moving random dots, were also affected by separation between the two comparison stimuli. The separation at which performance was affected in both tasks increased with retinal eccentricity in parallel with the increase in receptive-field size in neurons in cortical area MT. The loss with transfer of visual information between different spatial locations suggests a contribution of cortical areas with localized receptive fields to the performance of the memory task. The similarity in the spatial scale of the storage mechanism derived psychophysically and the receptive field size of neurons in area MT suggest that MT neurons are central to this task.

Cell-cycle, phase-specific activation of Maize streak virus promoters.

It is believed that geminiviral DNA replication is coupled to the cell-cycle regulatory complex of the plant cell and that the virus-early (complementary or C sense) gene products REP and REPA may be able to manipulate the regulation of the cycle. In this study, we examined expression from the promoters of Maize streak virus (MSV) in transgenic maize plants and cells to determine whether they showed cell-cycle specificity. Histochemical staining of plant roots containing "long and short" C-sense promoter sequences upstream of the GUS (beta-glucuronidase) reporter gene showed that promoter activity was restricted to the meristematic region of the roots and was enhanced by 2,4-dichlorophenoxy acetic acid (2,4-D) treatment. Analysis of reporter gene and cell-cycle-specific gene transcript levels coupled with flow cytometric data in synchronized transgenic maize cells revealed that all of the MSV promoters showed cell-cycle specificity. The coat protein gene promoter showed highest activity in early G2, whereas the C-sense promoter sequences produced two peaks of activity in the S and G2 cell-cycle phases.

Microstimulation of cortical area MT affects performance on a visual working memory task.

We applied electrical stimulation to physiologically identified sites in macaque middle temporal area (MT) to examine its role in short-term storage of recently encoded information about stimulus motion. We used a behavioral task in which monkeys compared the directions of two moving random-dot stimuli, sample and test, separated by a 1.5-s delay. Four sample directions were used for each site, and the animals had to indicate whether the direction of motion in the sample was the same as or different to the direction of motion in the test. We found that the effect of stimulation of the same directional column in MT depended on the behavioral state of the animal. Although stimulation had strong effects when applied during the encoding and the storage components of the task, these effects were not equivalent. Stimulation applied during the presentation of the sample produced signals interpreted by the monkeys as directional motion. However, the same stimulation introduced during the period of storage no longer produced signals interpreted as unambiguous directional information. We conclude that the directional information used by the monkeys in the working memory task is likely to be provided by neurons in MT, and the use of this information appears to be dependent on the portion of the task during which stimulation was delivered. Finally, the disruptive effects of stimulation during the delay suggest that MT neurons not only participate in the encoding of visual motion information but also in its storage by either maintaining an active connection with the circuitry involved in storage or being an integral component of that circuitry.

Long-term neurochemical changes after visual cortical lesions in the adult cat.

Peripheral deafferentation alters cortical function and such alterations have been shown to affect the cortical expression of the calcium-binding proteins calbindin and parvalbumin and of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). To determine whether cortical deafferentation produces similar effects, we examined the long-term consequences of cortical lesions on the neurochemistry of interconnected cortical areas. We studied the reciprocal effects of localized damage to either visual cortical areas 17 and 18, or posteromedial lateral suprasylvian (PMLS) cortex in the adult cat. These areas are strongly interconnected and play an important role in the processing of visual information. Combined lesions of areas 17 and 18 caused a marked, topographically specific decrease in the proportion of neurons expressing calbindin in supragranular layers of PMLS cortex. Similarly, lesions of PMLS cortex caused topographically restricted decreases in calbindin expression within supragranular layers of areas 17 and 18, but not in other cortical areas with which PMLS is interconnected. To categorize the calbindin-positive neurons affected by such lesions, we carried out double-labeling experiments for the inhibitory neurotransmitter GABA. This investigation showed lesions of areas 17 and 18 to affect calbindin-positive excitatory and inhibitory neurons equally, but PMLS lesions had stronger effects on inhibitory, calbindin-positive neurons. This finding may represent differential damage to feed-forward vs. feed-back projections in the two types of lesions. Finally, the expression of parvalbumin and GABA was unchanged, even in zones of decreased calbindin immunoreactivity. Our results suggest that damage to adult visual cortical areas, whether striate or extrastriate, induces neurochemical changes in the supragranular corticocortical network to which these areas belong. That changes were restricted to calbindin expression suggests cell-specific and/or biochemical pathway-specific alterations in calcium homeostasis.

The multiple roles of visual cortical areas MT/MST in remembering the direction of visual motion.

Although the role of cortical areas MT and MST (MT/MST) in the processing of directional motion information is well established, little is known about the way these areas contribute to the execution of complex behavioral tasks requiring the use of such information. We tested monkeys with unilateral lesions of these areas on a visual working memory task in which motion signals not only had to be encoded, but also stored for brief periods of time and then retrieved. The monkeys compared the directions of motion of two random-dot stimuli, sample and test, separated by a temporal delay. By increasing the temporal delay and spatially separating the two stimuli, placing one in the affected visual field and the other in the intact visual field, we were able to assess the contribution of MT/MST to specific components of the task: encoding (sample), retention (delay) and encoding/retrieval/comparison (test). We found that the effects of MT/MST lesions on specific components depended upon the demands of the task and the nature of the visual motion stimuli. Whenever stimuli consisted of random dots moving in a broad range of directions, MT/MST lesions appeared to affect encoding. Furthermore, when the lesions affected encoding of the sample, retention of the direction of stimulus motion was also affected. However, when the stimulus was coherent and the emphasis of the task was on the comparison of small direction differences, the absence of MT/MST had major impact on the retrieval/comparison component of the task and not on encoding or storage.

Attention increases sensitivity of V4 neurons.

When attention is directed to a location in the visual field, sensitivity to stimuli at that location is increased. At the neuronal level, this could arise either through a multiplicative increase in firing rate or through an increase in the effective strength of the stimulus. To test conflicting predictions of these alternative models, we recorded responses of V4 neurons to stimuli across a range of luminance contrasts and measured the change in response when monkeys attended to them in order to discriminate a target stimulus from nontargets. Attention caused greater increases in response at low contrast than at high contrast, consistent with an increase in effective stimulus strength. On average, attention increased the effective contrast of the attended stimulus by a factor of 1.51, an increase of 51% of its physical contrast.

Transient and permanent deficits in motion perception after lesions of cortical areas MT and MST in the macaque monkey.

We examined the nature and the selectivity of the motion deficits produced by lesions of extrastriate areas MT and MST. Lesions were made by injecting ibotenic acid into the representation of the left visual field in two macaque monkeys. The monkeys discriminated two stimuli that differed either in stimulus direction or orientation. Direction and orientation discrimination were assessed by measuring thresholds with gratings and random-dots placed in the intact or lesioned visual fields. At the start of behavioral testing, we found pronounced, motion-specific deficits in thresholds for all types of moving stimuli, including pronounced elevations in contrast thresholds and in signal-to-noise thresholds measured with moving gratings, as well as deficits in direction range thresholds and motion coherence measured with random-dot stimuli. In addition, the accuracy of direction discrimination was reduced at smaller spatial displacements (i.e. step sizes), suggesting an increase in spatial scale of the residual directional mechanism. Subsequent improvements in thresholds were seen with all motion stimuli, as behavioral training progressed, and these improvements occurred only with extensive behavioral testing in the lesioned visual field. These improvements were particularly pronounced for stimuli not masked by noise. On the other hand, deficits in the ability to extract motion from noisy stimuli and in the accuracy of direction discrimination persisted despite extensive behavioral training. These results demonstrate the importance of areas MT and MST for the perception of motion direction, particularly in the presence of noise. In addition, they provide evidence for the importance of behavioral training for functional recovery after cortical lesions. The data also strongly support the idea of functional specialization of areas MT and MST for motion processing.

Lesions in cat lateral suprasylvian cortex affect the perception of complex motion.

We examined the effects of bilateral ibotenic acid lesions of cat lateral suprasylvian (LS) cortex on motion perception. Cats were tested on tasks requiring integration of local directional signals, precise judgements of direction and extraction of structure-from-motion. All animals showed permanent deficits in integrating local motion signals. These deficits were most pronounced in the presence of directional noise and at larger spatial displacements. In addition, LS lesions produced a 2-fold loss in the accuracy of direction discrimination and large deficits in the perception of structure-from-motion. All of these losses were most severe during the first few weeks of testing following the lesion. These findings demonstrate that LS cortex plays an important role in the processing of stimuli requiring integration of motion information and limits the spatial scale over which such integration can proceed. Partial improvements in performance with time and/or training may be indicative of post-operative plastic changes in neurons outside of LS cortex.

The role of striate cortex in visual function of the cat.

We examined the contribution of area 17 to visual function in two cats whose fixation was monitored by means of scleral search coils. Ibotenic acid lesions were made within the physiologically identified representation of the lower left visual field of area 17. In a detection task in which the cats simply indicated the presence or absence of a vertical grating, contrast sensitivity loss was greatest at middle spatial frequencies with no loss in spatial resolution. However, when cats were required to discriminate between vertical and horizontal gratings, sensitivity loss was profound at both middle and high spatial frequencies with an octave loss of spatial resolution. This greater loss was not due to disrupted orientation discrimination since sensitivity to the orientation of coarse gratings was unaffected in the lesioned hemifield. We also found deficits in the ability to discriminate the direction of grating motion, but only at higher spatial and lower temporal frequencies. The role of area 17 in perceiving the global motion of complex patterns was also studied with high contrast, dynamic random dots drifting at high speeds. Paradoxically, area 17 lesion improved the perception of global motion. This improvement was eliminated by spatially filtering the dot patterns to remove high spatial frequencies, suggesting that the lesion has enhanced performance by interfering with masking by high spatial frequencies. Our results demonstrate that the performance of traditional detection tasks may be insensitive to the effects of area 17 lesions. Discrimination tasks, on the other hand, revealed that area 17 neurons play a major role in the perception of higher spatial frequency stimuli as long as they move or flicker at low rates, but contribute little to these functions when the stimuli are coarse and move at high speeds.

A reduction in the number of directionally selective neurons extends the spatial limit for global motion perception.

Dynamic random-dot targets were used to study neural mechanisms underlying motion perception. Performance of cats with severely reduced numbers of cortical directionally selective neurons (reduced DS) was compared to that of normal animals. We assessed the spatial properties of the residual motion mechanism by measuring direction discriminations at various dot displacements. At small displacements, reduced DS cats' motion integration thresholds for opposite direction discrimination were nearly normal. At larger displacements, their thresholds surpassed those of normal cats and their upper displacement limit (dmax) was increased by 0.35 deg. The accuracy of direction discrimination was reduced at small displacements, but at larger displacements direction difference thresholds of reduced DS cats approached or surpassed those of normals. These data were compared to the performance of humans who showed an extension of dmax for peripherally viewed targets. The data support the hypothesis that expansion in spatial scale of the motion mechanism may contribute to extension of dmax. Additional support for this hypothesis is provided by a modified direction discriminating line-element model. The model also suggests that changes in sampling of motion mechanisms in the reduced DS system may play a role.

Motion perception following lesions of the superior temporal sulcus in the monkey.

We examined the effect of bilateral ibotenic acid lesions, aimed at areas MT/MST in three macaques, on their perception of motion. The medial boundary of the lesions in the three monkeys was near the dorsal end of the STS, but the lesions extended different lengths ventrally along the STS. The lesions extended the shortest distance ventrally monkeys 1 and 2, covering most of MST but possibly sparing a portion of lateral MT. That in monkey 3 damaged all of MT and MST bilaterally and extended through most of FST. All three lesions caused a temporary disruption, followed by at least partial recovery, of most motion thresholds. Permanent effects of the lesions on visual sensitivity were graded with lesion extent. Contrast sensitivity for detecting low-spatial-frequency (1 cycle/degree) drifting gratings over a wide range of drift rates, as well as for identifying their direction of motion, was slightly affected only in monkey 3. Only monkeys 2 and 3 showed a deficit in discriminating stimulus speed, and the size of the loss was two- to fourfold. Discrimination of opposite directions of dot pattern motion, which required integration of local motion signals, was mildly affected in monkeys 2 and 3, and not affected in monkey 1. However, addition of directional noise to this discrimination caused the performance of all monkeys to be permanently disrupted, especially that of monkeys 2 and 3. Finally, direction difference thresholds were elevated by a factor of 2-4 after the lesions in all three monkeys. Many of these deficits were more pronounced during the first 2 months of testing following the lesion. Thus, our results demonstrate that areas within dorsal STS make an important contribution to the performance of various motion perception tasks including the discrimination of small differences in direction and speed, and the perception of global motion in the presence of directional noise. The residual motion perception, even in the monkey with virtually complete removal of areas MT/MST, may suggest either that these tasks are normally mediated in part by cortical areas outside of areas MT and MST, or that the disrupted functions were partially assumed by other cortical areas after lesions.

Spatiotemporal sensitivity following lesions of area 18 in the cat.

The contribution of cat area 18 to spatiotemporal sensitivity and to motion processing was assessed in cats with unilateral ibotenic acid lesions placed in physiologically identified portions of area 18. The lesions were centered in the representation of the lower right visual field, about 10 degrees from the vertical meridian. In one of the animals, the lesion invaded a small portion of area 19. We measured detectability of various spatiotemporal stimuli placed within the lesioned and intact portions of the visual field, while monitoring eye position with a scleral search coil. We found a loss of sensitivity to gratings of low and intermediate spatial frequency, within the ablated portion of the visual field. The sensitivity loss was 0.6-1.0 log units at low and intermediate spatial frequencies, and decreased at higher frequencies with the resolution limits remaining intact. The loss extended over a range of temporal frequencies for both drifting gratings and grating modulated in counterphase. We also found that within the lesioned hemifield, the cats were unable to discriminate between rightward and leftward motion even at the highest contrasts. These results demonstrate that area 18 plays an important role in detecting drifting low- and intermediate-spatial-frequency targets and is likely to represent a critical stage in the cortical processing of motion signals.

Spatial vision of the cat: variation with eccentricity.

We examined the grating acuity and contrast sensitivity of cats whose eye position was monitored with a scleral search-coil technique. During each trial, the cat was required to maintain fixation on a laser spot and respond to the presence or the absence of a grating by pressing a right or left pedal. With this procedure, acuity was measured along the horizontal and vertical meridian over a range of eccentricities in the nasal, temporal, superior, and inferior retina. In addition, contrast sensitivity for stationary and drifting gratings was measured for the temporal retina along the horizontal meridian. Acuity in area centralis reached about 3.5 cycle/deg and declined by 0.5 octaves at 4 deg and by about 1.3 octaves at 16-deg eccentricity in the nasal retina. The acuity was higher in the nasal than temporal retina. At all eccentricities, spatial resolution exceeded the resolution limit derived from Y (alpha)-cell properties. Contrast sensitivity also decreased as the eccentricity increased when the target size was held constant. The slope of sensitivity-eccentricity function was relatively shallow for a low spatial frequency (0.30 cycle/deg) with sensitivity decreasing by a factor of 1.5-2 at 8-deg eccentricity. The slope of the sensitivity falloff for high spatial-frequency gratings (1.2 cycle/deg) was steeper, with a 5-10-fold difference in sensitivity between 0 and 8 deg. By varying the target size, we determined that the summation area in the cat is about a factor of 3 smaller in area centralis than a 16-deg eccentricity.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of directionally selective neurons in the perception of global motion.

Dynamic random dot targets consisting of many localized motion vectors have been used to study the pooling of local motion signals into a global motion percept (Williams and Sekuler, 1984). In such displays, the dots are displaced with a constant step size and the direction of motion for each dot is chosen at random from a specified distribution. When the distribution extends over 360 deg, the display consists only of local random motion of individual dots and no coherent motion is reported. However, when the distribution is less than 360 deg (biased), the stimulus appears to flow in a single direction. We examined the effects of reducing the number of directionally selective (DS) cortical neurons on this integration process. Normal cats and cats with severely reduced proportions of DS neurons were trained on 2 direction discrimination tasks. The discrimination of opposite directions was examined while varying either the range of directions of local motion, or the proportion of dots moving with biased distribution. When all dots in the display were directionally biased, cats with reduced numbers of DS neurons performed the task as well as normal cats and humans (threshold range: 280-320 deg). However, when the proportion of biased dots decreased, these animals had severe deficits. Thus, in the absence of noise, even a very small number of DS neurons can perform spatial pooling of local directional signals, and support normal discrimination of opposite directions. However, a full complement of directional detectors appears necessary when the motion signal is masked by noise. The discrimination of small differences in direction revealed far more severe deficits, even when all the dots in the display were directionally biased (no noise).(ABSTRACT TRUNCATED AT 250 WORDS)

Deficits in speed discrimination following lesions of the lateral suprasylvian cortex in the cat.

We examined the role of the lateral suprasylvian (LS) cortex in motion perception by testing the ability of three cats to detect moving targets and to discriminate differences in stimulus direction and speed before and after making bilateral ibotenic acid lesions in LS. The lesions had little or no effect on contrast sensitivity for detecting moving sinusoidal gratings. Moreover, we found no deficits in discriminating opposite directions of motion: the cats discriminated grating directions at threshold contrasts. All three cats, however, showed permanent deficits in discriminating differences in speed and in flicker rate. The deficits were most pronounced at higher temporal and spatial frequencies and at lower contrasts. This result suggests that LS plays an important role in the analysis of stimulus speed. It appears that information needed for discriminating opposite directions of motion may be signalled by visual areas outside LS.

Effects of serial lesions of telencephalic components of the visual system in pigeons.

A serial-lesion technique was used to investigate interactions in visual processing between telencephalic components of the pigeon visual system. Pigeons were trained to discriminate pairs of stimuli that differed in color, intensity or pattern. After mastering the discrimination tasks, they were assigned to one of three groups. The first group (WI-EII) received lesions of the visual Wulst and were retested. After the discrimination tasks were again mastered, a second set of lesions was made, this time in the ectostriatum. The birds were tested once again after the second surgery. The second group (EI-WII), underwent the same sequence of events except that the order of the lesions was reversed. In the third group (E + W), lesions of both the visual Wulst and ectostriatum were made in a single operation, followed by retesting. The performance after the first lesion of the subjects in each of the two-stage lesion groups was typical of performance after such lesions; i.e. the birds with visual-Wulst lesions showed little or no impairment on any of the tasks, whereas the pigeons with ectostriatum lesions showed considerable deficits in intensity and pattern discrimination, which diminished after prolonged retraining. In contrast, the pigeons in the one-stage group (E + W) showed profound deficits that appeared to be permanent. The performance after the second operation of the WI-EII group was the same as that of pigeons with lesions of ectostriatum alone; i.e. destruction of ectostriatum first or second resulted in the same duration of impairment. The performance of the EI-WII group after its visual Wulst lesion, however, was similar to that observed in the E + W group. The results are interpreted as a reflection of parallel processing within the avian visual system; i.e. the presence of an intact tectofugal pathway may mask the effects of thalamofugal pathway interruption.

Discrimination of differences in speed and flicker rate depends on directionally selective mechanisms.

The present study compared discriminations of differences in speed to differences in temporal frequency and examined the role of directionally selective mechanisms in such discriminations. In measuring the contrast dependence of speed and temporal frequency discriminations two different techniques were used to reduce the role of directionally selective mechanisms. The first was the virtual elimination of directional selectivity in the visual cortex of cats by stroboscopic rearing. The second was the reduction of directional sensitivity in normal humans and cats by testing with gratings of high spatial and low temporal frequency. Discrimination of the temporal frequency of sinusoidal gratings flickered in counterphase was worse than discrimination of speeds of moving gratings. Under conditions that maximize the sensitivity of directional mechanisms (low spatial, moderate temporal frequency) Weber fractions for speed and flicker in all normal observers (cats and humans) were constant at higher contrast and increased only as contrast began to approach threshold. In strobe-reared cats sensitivity for direction was 10 times lower than sensitivity for detection. They were able to discriminate speeds and temporal frequencies only at contrasts that exceeded contrast threshold for direction. This was also true for a normal cat whose sensitivity for direction was reduced by increasing the spatial frequency of the grating. In all cases Weber fractions for flicker as a function of contrast were greater than but paralleled those for speed.