The connectivity underlying the orientation selectivity in the infragranular layers of monkey striate cortex.

The striate cortex of the monkey was studied with an array of 30 closely spaced microelectrodes. Responses to oriented light bars were recorded simultaneously, and interneuronal connectivity was inferred from spike train correlations of pairs of neurones. Between cells with parallel preferred orientations, interactions were strong at short ranges but negative or only weakly positive at separations of 0.3-0.4 mm. At these distances, the stronger correlations were observed between orthogonally oriented cells. The finding does not explain the generation of orientation selectivity, but it is in agreement with a cooperative principle explaining the spatially regular arrangement of oriented cells.

Effects of septal and/or raphe cell suspension grafts on hippocampal choline acetyltransferase activity, high affinity synaptosomal uptake of choline and serotonin, and behavior in rats with extensive septohippocampal lesions.

At 31 days of age, Long-Evans female rats sustained aspirative lesions of the septohippocampal pathways and, 14 days later, received intrahippocampal suspension grafts prepared from the region including the medial septum and the diagonal band of Broca (Group S, n = 11), from the region including the mesencephalic raphe (Group R, n = 11) or from both regions together (Group S+R, n = 11). Sham-operated (Group Sham, n = 9) and lesion-only (Group Les, n = 11) rats served as non-grafted controls. Seven Sham, 7 Les and 8 rats from each transplant group were tested for home cage activity (6 months after grafting) and radial maze performance (between 7.5 and 8.5 months post-grafting). One month after completion of behavioral testing, the dorsal hippocampi of these rats were prepared for measuring choline acetyltransferase (ChAT) activity and high affinity synaptosomal uptake of both [3H]choline and [3H]serotonin. The remaining rats were used for histological verifications on brain sections stained for acetylcholinesterase (AChE). The lesions increased locomotor activity, impaired radial maze learning and, in the dorsal hippocampus, reduced AChE positive staining, decreased ChAT activity (-73%) as well as high affinity uptake of both choline (-81%) and serotonin (-82%). Neither type of transplant produced any significant behavioral recovery. However, septal transplants increased hippocampal AChE positivity, restored ChAT activity and enhanced choline uptake to 116% and 70% of the values found in sham-operated rats, respectively; they had no significant effect on uptake of serotonin. Transplants from the raphe region had weak effects on hippocampal AChE positivity, increased both the ChAT activity and the choline uptake to 70% ad 38% of the sham-operated rats, respectively, and produced an (over)compensation of the serotonin uptake which reached 324% of the values found in sham-operated rats. The co-transplantation of both regions resulted in restoration of ChAT activity (113% of sham-operated rats values), choline uptake (83% of sham-operated rats) and serotonin uptake (129% of sham-operated rats). Our neurochemical data show that after extensive denervation of the hippocampus, intrahippocampal grafts of fetal neurons may foster a neurotransmitter-specific recovery which depends upon the anatomical origin of the grafted cells: a graft rich in serotonergic neurons overcompensates the serotonergic deficit, a graft rich in cholinergic neurons attenuates the cholinergic deficit, whereas a mixture of both types of grafts produces recovery from both types of deficits. Thereby, both the feasibility and the interest of the co-grafting technique are confirmed.(ABSTRACT TRUNCATED AT 400 WORDS)

Rhythmic and patterned neuronal firing in visual cortex.

We present evidence from multi-electrode recordings that the stimulus driven activity in a region of monkey visual cortex displays synchronous, oscillatory behaviour with temporally changing patterns of firing. The spatial-temporal patterns are dependent on cortical layer and on visual stimulus, 'build up and die down' with a period of 70-100 msec, and cannot be factorized into separate spatial and temporal components. This finding supports neural network models which explicitly incorporate spatial-temporal structure, ruling against a purely spatial code for information processing. The new type of analysis presented here is also suitable for direct on-line interactive analysis during multi-electrode recording experiments. Further, it could be modified for use in human EEG or evoked response recordings.

Multimicroelectrode investigation of monkey striate cortex: spike train correlations in the infragranular layers.

1. In the infragranular layers of the striate cortex of three monkeys, we studied tangential neuronal interactions by analyzing cross-correlograms calculated from spike trains recorded with 30 closely spaced microelectrodes. 2. There are two major types of correlogram structures--"narrow" peaks a few milliseconds wide, sometimes accompanied by small lateral troughs, and "broad" peaks approximately 30- to 100-ms wide. Isolated troughs are rare. Both types of structures are superimposed in the same correlograms; they are not due to shared optical stimulation. 3. In layer VI, narrow peaks are largest in a short lateral range of approximately 220 micron, and they depend on ocularity. In layer V, the lateral range is greater, and the dependency on ocularity is weak. 4. In addition, narrow peaks are largest at distances of 160 micron if the angles of preferred orientation are similar. In layer VI, however, at tangential distances of 300-400 micron, peaks are smaller, and troughs are found more often, for neuron pairs with parallel orientations compared with those with orthogonal orientations. From the agreement of this finding with a cooperative theory, we conclude that orientation selectivity is shaped by collective interactions. 5. Broad peaks always depend on ocularity, and the associated lateral interaction range exceeds the maximum of 1 mm investigated. Their size sharply decreases with receptive-field distance. 6. Average mutual delays of spikes of neuron pairs, manifest as lateral displacements of broad peaks, are interdependent; the delay between neurons 1 and 3 is the sum of that of neurons 1 and 2 and of neurons 2 and 3. This feature permits to rank the neurons on a "delay scale." 7. We conclude from 5 and 6 above that broad peaks partly result from intraretinal interactions whose effects are transmitted to the cortex via slow and fast pathways. 8. Lateral troughs adjacent to narrow peaks provide evidence that neurons at the "slow" end of the delay scale inhibit those at the "fast" end, and to a lesser extent, nondirectional neurons inhibit directional ones.

Neuronal synchrony in monkey striate cortex: interocular signal flow and dependency on spike rates.

A hypercolumn of monkey striate cortex was studied with an array of 30 closely spaced microelectrodes. Prominent broad peaks appearing in spike train correlograms are considered here. These were not due to shared stimulation, were mostly 30 to 100 ms wide, and were presumably the consequence of intraretinal lateral interactions. The correlogram peak areas were found to be predictable from the products of the spike rates, to which they were proportional. One can conclude that the correlation occurs before the overall reduction of spike rates from retina to cortex takes place. Furthermore, when a neurone dominated by one eye was stimulated via that eye, the correlogram formed with a neurone dominated by the other eye showed a displaced peak, indicating that excitation traveled from the well-responding to the unresponsive neurone in about 10 ms. When a left-eye stimulus was delivered, the same pair of neurones had a correlogram with a reversed peak displacement. This effect was only observed in layers IVb and c, indicating that in these layers the paths from the two eyes to a given cell are of unequal length, whereas in other layers, cells receive input from both eyes via similar connections differing only in strength.

Saccadic eye movements of dyslexic and normal reading children.

Twenty-four children made saccades in five noncognitive tasks. Two standard tasks required saccades to a single target presented randomly 4 deg to the right or left of a fixation point. Three other tasks required sequential saccades from the left to the right. 75 parameters of the eye-movement data were collected for each child. On the basis of their reading, writing, and other cognitive performances, twelve children were considered dyslexic and were divided into two groups (D1 and D2). Group statistical comparisons revealed significant differences between control and dyslexic subjects. In general, in the standard tasks the dyslexic subjects had poorer fixation quality, failed more often to hit the target at once, had smaller primary saccades, and had shorter reaction times to the left as compared with the control group. The control group and group D1 dyslexics showed an asymmetrical distribution of reaction times, but in opposite directions. Group D2 dyslexics made more anticipatory and express saccades, they undershot the target more often in comparison with the control group, and almost never overshot it. In the sequential tasks group D1 subjects made fewer and larger saccades in a shorter time and group D2 subjects had shorter fixation durations than the subjects of the control group.

Dead zone for express saccades.

The saccadic eye movements of three humans and one non-human primate (a male rhesus monkey) have been measured for target eccentricities between 0.3 and 15 deg. With a gap task (fixation point offset precedes target onset by 200 ms) and a target at 4 deg, all subjects produced reasonable amounts of express saccades as indicated by a clear peak in the distribution of their saccadic reaction times (SRT): about 100 ms in human subjects and 70 ms in the monkey. This peak disappeared with decreasing target eccentricity below 2 deg, but saccades of longer (regular) reaction times were still present. Thus it was found that there exists a dead zone for express saccades. In addition, small saccades have a much stronger tendency to overshoot the target and their velocity falls above the main sequence as defined by the least square fit of an exponential v = vo(1-exp(-a/ao)) to the maximal velocity (v) versus amplitude (a) relationship (vo and ao are constants fitted). It is concluded that for small saccades the express way is blocked functionally or does not exist anatomically.

Occurrence of express saccades under isoluminance and low contrast luminance conditions.

Saccadic reaction times (SRTs) of three human subjects were analyzed. The gap paradigm was used (i.e. fixation point offset precedes target onset) to obtain high proportions of express saccades (i.e. saccades of extremely short reaction times) in the SRT distributions. In one set of experiments, the luminance of the (red) saccade target was varied from brighter to darker than the (green) background including an isoluminance condition. Express saccades were obtained in response to pure color contrast stimuli with about the same frequency and reaction time as to stimuli with both color and luminance contrast. In a second experiment, the luminance contrast of a white target on a white background was lowered below 10%. Again the number of express saccades was not reduced. Thus, in contrast to other perceptual phenomena the visual neural mechanisms underlying the generation of express saccades are not affected by isoluminance nor low contrast luminance.

Separate populations of visually guided saccades in humans: reaction times and amplitudes.

The saccadic eye movements of 20 naive adults, 7 naive teenagers, 12 naive children, and 4 trained adult subjects were measured using two single target saccade tasks; the gap and the overlap task. In the gap task, the fixation point was switched off before the target occurred; in the overlap task it remained on until the end of each trial. The target position was randomly selected 4 degrees to the left or 4 degrees to the right of the fixation point. The subjects were instructed to look at the target when it appeared, not to react as fast as possible. They were not given any feedback about their performance. The results suggest that, in the gap task, most of the naive subjects exhibit at least two (the teenagers certainly three) clearly separated peaks in the distribution of the saccadic reaction times. The first peak occurs between 100 and 135 ms (express saccades), the second one between 140 and 180 ms (fast regular), and a third peak may follow at about 200 ms (slow regular). Other subjects did not show clear signs of two modes in the range of 100 to 180 ms, and still others did not produce any reaction times below 135 ms. In the overlap task as well three or even more peaks were obtained at about the same positions along the reaction time scale of many, but not all subjects. Group data as well as those of individual subjects were fitted by the superposition of three gaussian functions. Segregating the reaction time data into saccades that over- or undershoot the target indicated that express saccades almost never overshoot. The results are discussed in relation to the different neural processes preceding the initiation of visually-guided saccades.