Stereopsis in the falcon.

Stereoscopic depth perception is demonstrated in the falcon, a non-mammalian with binocular vision. This result complements recent physiological evidence for binocular interaction in the bird visual system, and suggests that stereopsis may be a general attribute of vertebrate vision and not an exclusive product of mammalian evolution.

Falcon visual acuity.

Grating acuity, the ability to resolve high-contrast square-wave gratings, was measured in a falcon and in humans under comparable conditions. This behavioral test of falcon acuity supports the common belief that Falconiformès have superb vision-the faclon's threshold was 160 cycles per degree, while the human thresholds were 60 cycles per degree. Falcon acuity, however, was much more dependent on lumanance, declining sharply with decreases in luminance.

Electroretinograms and visual evoked potentials in long-term monocularly deprived cats.

The effects of long-term monocular lid-suture deprivation on visual-evoked cortical potentials (VEPs) and flash- and pattern-evoked electroretinograms (FERGs and PERGs, respectively) were assessed in the cat. VEPs were virtually eliminated when recorded with the deprived eye, indicating that the lid suture produced a severe amblyopia in that eye. In contrast, FERGs and PERGs were more similar for both deprived and nondeprived eyes and comparable to those recorded in normal animals. The current findings demonstrate that long-term deprivation (3-4 yr) does not produce systematic changes in the electroretinogram.

Spatial contrast sensitivity of monocularly deprived cats after removal of the non-deprived eye.

Spatial contrast sensitivity was behaviorally determined for two monocularly lid-sutured cats. The spatial contrast sensitivity function for the non-deprived eyes matched previously reported functions obtained from normally reared cats. In comparison, sensitivity was significantly lower at all spatial frequencies for the deprived eye of both cats. In one cat, subsequent removal of the non-deprived eye resulted in a two to threefold increase in sensitivity at all spatial frequencies. However, no improvement in sensitivity was observed for the second animal. Single-unit electrophysiological recording in the striate cortex of these two animals revealed a postenucleation difference in the percent of visually influenced cells. In the cat for which no behavioral improvement occurred, only 13% of striate cortex cells could be driven by visual stimulation of the previously deprived eye. In contrast 38% of striate cortex cells were similarly influenced in the cat for which visual improvement was observed. These data suggest a correlation between the physiological effectiveness of the postcritical period enucleation procedure and the visual capacity of monocularly lid-sutured cats.

Cross-species correspondence of spatial contrast sensitivity functions.

Spatial contrast sensitivity functions (CSFs) have been obtained for at least 9 species, including man. In the present paper, the shapes and octave band widths of these functions are compared. For most species, the shape of the CSF was an inverted-U, and the full width at half amplitude of the CSFs varied less than one octave. These similarities suggest that there is a close correspondence of the CSFs of these diverse animals; the major difference is the location of each CSF in the spatial frequency domain.

The effects of monocular deprivation on the visual latency of geniculate X- and Y-cells in the cat.

Monocular lid suture deprivation during early visual development of the cat alters the temporal flow of retinal information as it passes through the dorsal lateral geniculate nucleus. The latencies of Y-cells located in the deprived layers and of both X- and Y-cells in the non-deprived layers are shorter than the latencies of their counterparts in normally reared cats. The visual response onset latencies of X-cells located in the deprived geniculate layers lag those of X-cells in the non-deprived layers.

Effect of depth separation on metacontrast masking.

In most investigations of visual masking, spatially adjacent contours are located in the same depth plane, a practice derived from the assumption that depth is not a critical variable in contour formation. In nine experiments, the dependence of masking on the relative depth positions of the target and mask was examined. To permit facile manipulation of depth without introducing confounding changes in proximal stimulation, the stimuli were stereoscopic contours formed from dynamic random-element stereograms generated as anaglyphs on a projection color-television display. The target was a briefly presented Landolt C with gap position randomly varied; the mask was an annulus that surrounded the target. Masking with stereoscopic contours was robust and subject to the same spatial and temporal variables influential in masking with physical contours. Depth separation of the target and mask produced significant reductions in masking and interacted with temporal order and egocentric stimulus position. These results have implications for models of visual masking and indicate that perceived depth is important in contour formation.

Spatial displacement sensitivity of X- and Y-cells in the dorsal lateral geniculate nucleus of the cat.

The sensitivity of X- and Y-cells in the dorsal lateral geniculate nucleus of the cat to small, temporally modulated displacements of grating stimuli was measured at 0.175, 0.25, 0.50, 1.00, and 2.00 c/deg. For every cell, two threshold measures were determined: first, a contrast threshold with a counterphase grating and then a displacement threshold with a grating matched in spatial frequency, but whose contrast was 2.5 times the threshold value. The results showed that displacement thresholds of both X- and Y-cells decreased with increasing spatial frequency. At low spatial frequencies, mean displacement thresholds of X- and Y-cells were similar, but at intermediate spatial frequencies, Y-cell thresholds were lower than X. X-cell displacement thresholds were lower than Y only at the highest spatial frequency tested. Consistent with previous reports, contrast thresholds also varied with spatial frequency for both X- and Y-cells. The local luminance differences produced by the contrast threshold and displacement threshold stimuli for the two classes of cells were compared. Across all spatial frequencies, the change in position of the gratings at displacement threshold produced smaller luminance differences than the counterphase gratings at contrast threshold. This enhanced sensitivity of X- and Y-cells to a local luminance changes produced by grating displacement was related to the high spatial contrast of the grating and not to the displacement per se.

Visual response latency of X- and Y-cells in the dorsal lateral geniculate nucleus of the cat.

Visual response latencies and rise times of X- and Y-cells in the dorsal lateral geniculate nucleus (dLGN) of anaesthetized, paralyzed cats were measured during repeated stimulation with sinusoidal grating patterns. Measurements were restricted to individual stimulus trials on which the instantaneous discharge rate exceeded a criterion amplitude defined in terms of the baseline activity of each cell. The latencies of response onsets and response peaks were systematically related to the spatial frequency and contrast of the grating stimuli. Response latencies of Y-cells were shortest for gratings of low spatial frequency (0.17 c/deg) and increased monotonically with increases in spatial frequency. Response latencies of X-cells were shortest for gratings of intermediate spatial frequency (0.75 c/deg) and longer for lower and higher spatial frequencies. Latencies decreased monotonically with increases in stimulus contrast from 5 to 40% for both X- and Y-cells. In general, short-latency responses were less variable than long-latency responses. This was true for absolute as well as relative measures of variability. The mean onset and peak latencies of Y-cell responses were 10-15 msec shorter than the corresponding latencies of X-cell responses to stimuli of optimal spatial frequency and contrast. The rise times (latency of response peak minus latency of response onset) of Y-cell responses were consistently shorter than those of X-cells in spite of the higher peak responses of Y-cells. The results of this study are consistent with the idea that low spatial frequency information is passed through the lateral geniculate nucleus more quickly than is high spatial frequency information. These data provide support for models of visual processing wherein a coarse, global analysis of the visual scene by Y-cells precedes a finer, local analysis by X-cells.

An oblique effect of spatial summation.

Contrast thresholds for sinewave gratings were measured as a function of the number of cycles in the grating for two different orientations and three spatial frequencies. At high spatial frequencies, where an oblique effect of contrast sensitivity was demonstrated, there was a greater increase in contrast sensitivity as a function of the number of cycles for the vertical gratings than for oblique gratings. At a lower spatial frequency, where the contrast sensitivity anisotropy was greatly diminished, both orientations displayed a similar increase in contrast sensitivity when the number of cycles in the gratings was increased.

Visual latency of ganglion X- and Y-cells: a comparison with geniculate X- and Y-cells.

Visual response latencies and rise times of X and Y ganglion cells recorded in the optic tract of anaesthetized, paralyzed cats were measured during repeated stimulation with sinusoidal gratings. These measures were compared with visual latencies and rise times of X- and Y-cells in the dorsal lateral geniculate nucleus. Measurements were restricted to individual trials on which the instantaneous discharge rate exceeded a criterion amplitude defined in terms of the statistics of the baseline activity of each cell in order to screen out false alarm responses. The onset and peak latencies of ganglion Y-cells are about 10-15 msec shorter than those of ganglion X-cells at low spatial frequencies (less than 0.25 c/deg) but about 10-20 msec longer at higher spatial frequencies (greater than 0.75 c/deg/). The onset latencies of geniculate X- and Y-cells lag their ganglion counterparts by 10-20 msec. Despite a delay in onsets of geniculate responses, the peak latencies of geniculate and ganglion X-cells are similar, and peak latencies of geniculate Y-cells are even shorter than those of their ganglion inputs. The short latencies of the peak responses of geniculate Y-cells are related to their short response rise times. A functional consequence of the bursty, but fast responses of geniculate Y-cells may be to accelerate the processing of lower spatial frequencies by the retino-geniculate Y-cell pathway.

Visual attention mechanisms show a center-surround organization.

The sudden onset of a cue triggers visual attention, which then enhances visual processing in the zone near the cue. This enhancement causes a motion illusion in subsequent stimuli presented near the cue. At greater separations from the cue, the illusory motion reverses direction, indicating prolonged processing speed. Measurements of the strength and direction of illusory motion at increasing separations from the cue reveal an attentional 'perceptive field' with an excitatory center at the locus cued and an inhibitory surround subtending the remaining visual field. These findings help explain the traditional attentional 'benefits' and 'costs' of attention.

Transient visual attention is dominated by the magnocellular stream.

Robust visual attentional responses are produced by the sudden onset of a visual cue, but the properties of cues that best elicit an attentional response are not fully known. We used the line-motion illusion (Hikosaka et al., 1991) to investigate the optimal cue properties that evoke visual attention. We found that visual attention is driven primarily by the luminance contrast of the cue. Furthermore, by manipulating the spatial, chromatic, and contrast properties of cues, we found that magnocellular (M) stream biased cues always override the response to parvocellular (P) stream biased cues, even when the P stream biased cues are presented first. Our data suggest that cues that preferentially excite the M pathway predominantly capture visual attention.

Disparity tuning of the stereoscopic (cyclopean) motion aftereffect.

Across five experiments this study investigated the disparity tuning of the stereoscopic motion aftereffect (adaptation from moving retinal disparity). Adapting and test stimuli were moving and stationary stereoscopic grating patterns, respectively, created from dynamic random-dot stereograms. Observers adapted to moving stereoscopic grating patterns presented with a given disparity and viewed stationary test patterns presented with the same or differing disparity to examine whether the motion aftereffect is disparity contingent. Across experiments aftereffect duration was greatest when adapting motion and test pattern both were presented with zero disparity and in the plane of fixation. Aftereffect declined as disparity of adapting motion and/or test pattern increased away from fixation, even under conditions in which depth position of adapt and test was equal. This argues against a relative depth separation explanation of the decline, and instead suggests that the amount of adaptable substrate decreases away from fixation.

Temporal modulation of the background affects the sensitization response of X- and Y-cells in the dLGN of cat.

The responses of X- and Y-cells to a small flashing test probe modulated at 2 Hz were measured as a function of the diameter of a concentric circular background. The background was either a static homogeneous disk, a flickered homogeneous disk, a static radially-vaned disk, or a rotating vaned disk, all of equivalent space- and time-averaged luminance. Variation of background diameter produced systematic changes in response that reflected general X- and Y-cell receptive field differences. Initial increases in background diameter decrease the response to the test probe ("desensitization") of both X- and Y-cells to a minimum level at a diameter that approximates the size of the receptive field center. Further increases in the background diameter result in an increase of the response to the test probe ("sensitization") that is very large for X-cells and much less for Y-cells. Temporal modulation of the background does not alter the desensitization of X- or Y-cells, but strongly reduces the sensitization of X-cells. Temporal modulation of the background by flicker and by motion produced equivalent effects. These results are compared to similar human psychophysical tests used clinically. Implications concerning the mediation of psychophysical sensitization are also considered.