Hide-Aki Saito

The relationship between response characteristics to flicker stimulation and receptive field organization in the cat's optic nerve fibers

Abstract The response characteristics of cats optic nerve fibers to flicker stimulation were investigated. As the flicker frequency increases, the average impulse frequency in Type I-fibers (phasic type) increases, passes through a maximum, and finally falls off. When the flicker frequency is adjusted properly, Type I-fiber which has a distinct surround in its receptive field shows a repetitive firing at high frequency (about 200 Hz). The average impulse frequency in Type II-fibers (tonic type) remains almost unchanged over a wide range of flicker frequencies.

Hippocampal LTP depends on spatial and temporal correlation of inputs

We studied the LTP inducing factors using temporally and spatially modulated stimuli given to the hippocampal neural network. It was found that when the spatial factors were maintained to be constant the positive correlation in the successive inter-stimulus intervals contributes to produce larger LTP. On the other hand, if the temporal factors are kept constant, the spatial coincidence contributes to produce larger LTP. We propose a learning rule by which these experimental results can be consistently interpreted. Copyright 1996 Elsevier Science Ltd.

Pharmacological and morphological differences between X- and Y-type ganglion cells in the cat's retina

Pharmacological and morphological differences between X- and Y-cells of the cats retina were studied using extracellular as well as intracellular recordings of the ganglion cells in the perfused eye-cup preparations. First, the effects of strychnine and bicuculline on the center and the surround responses were investigated. Strychnine blocked the surround inhibition of on-center X-cells, whereas bicuculline blocked that of on-center Y-cells, suggesting that these two-types of cells have different inhibitory interneurons which employ different neurotransmitters. In contrast, the center and the surround responses of off-center cells were reduced by bicuculline, leaving brief transient excitations, irrespective of whether the cells were X- or Y-type. Second, cells whose responses were studied intracellularly and classified as X- or Y-type, were stained with Lucifer yellow CH and observed in whole-mount preparations. It was found that X-cells have morphological characteristics of beta-cells, and Y-cells those of alpha-cells.

MT neurons in the macaque exhibited two types of bimodal direction tuning as predicted by a model for visual motion detection

We previously proposed a model for detecting local image velocity on the magnocellular visual pathway (Kawakami & Okamoto (1996) Vision Research, 36, 117-147). The model detects visual motion in two stages using the hierarchical network that includes component and pattern cells in area MT. To validate the model, we predicted two types of bimodal direction tuning for MT neurons. The first type is characteristic of component cells. The tuning is bimodal when stimulated with high-speed spots, but unimodal for low-speed spots or for bars. The interval between the two peaks widens as the spots speed increases. The second type is characteristic of pattern cells. The tuning is bimodal when stimulated with low-speed bars, but unimodal for high-speed bars or for spots. The interval widens as the bars speed decreases. To confirm this prediction, we studied the change of direction tuning curves for moving spots and bars in area MT of macaque monkeys. Out of 35 neurons measured at various speeds, six component cells and four pattern cells revealed the predicted bimodal tunings. This result provided neurophysiological support for the validity of the model. We believe ours is the first systematic study that records the two types of bimodality in MT neurons.

Gain control mechanisms in X- and Y-type retinal ganglion cells of the cat

A change in responsiveness caused by a spot of light (conditioning spot, CS; 3 sec in duration) presented within a central region of the receptive field of X- and Y-type retinal ganglion cells of the cat was investigated by measuring the magnitude of responses to another spot of light (test spot, TS; 50 msec in duration) which was juxtaposed with the CS within the same receptive fields central region. Responses to the TS were suppressed steadily during the on-phase of the CS as if it were divided by a certain value. This fact indicates that the gain of the center mechanism was changed by the CS presentation. The setting of the gain to a new level was rapid (within 100 msec after the onset or the cessation of the CS), and the magnitude of a gain change was not affected by the surround antagonism. These characteristics of the gain control were common to X- and Y-cells under both mesopic and scotopic levels of light adaptation.

Gain control mechanisms within the receptive field center of cats retinal ganglion cells

In the previous studies (Fukada, 1971; Fukada and light (5’ dia) were presented on a tangent screen which Saito, 1971X the cat’s retinal ganglion cells have been was placed 1.37m in front of the cat’s eyes and illuclassified into Type-I and Type-II, independently of minated at 59 cd/m2 by distant fluorescent tubes. The onor off-center subdivision, on the basis of the TS was presented at the center of the RF (the most receptive field proper&s and the axonal conduction sensitive part of the center region) and CS was located velocities. Since then, accumulated data by us (&to, 15’ eccentric to TS. The CS was still well within the Shimahara and F&da, 1970, 1971) and others (Clecenter region of the RF. In some experiments, the land, Dubin and Levi& 1971; Hoflinann, Stone and positions of TS and CS were interchanged with essenSherman, 1972; Ikeda and Wright, 1972; Cleland, tially the same results. The CS was presented once Levick and San&son, 1973) suggest that our Type-I every 12 set and lasted for 3 sec. The TS was flashed cells correspond to Y-cells of Enroth-Cugell and Robfor 100 or 200 msec with a variable delay relative son (1966) and Type-II to X-&Is. In this paper, X/Y to the onset or the cessation of CS. The luminance terminology will be used because of its familiarity. of the spots of light, which were maximally available It was observed that the response of on-center Xat 3.9 x ld and 6.8 x 103 cd/m2 for TS and CS cell to a flashing spot of light presented at the center respectively, were adjusted with neutral density filters of the receptive field (RF) was gradually suppressed so that an algebraic sum of the responses to TS and with successive stimulation. The suppression of the CS was well within the dynamic range of the cell’s response was aIso observed even when the preceding response. To focus the spot stimuli on the retina and spots had been presented at a little diierent position to keep high image quality, a contact lens (usually, of the center region. By contrast, the response of Y+ 10 D) with 4mm aperture was used. cell remained constant for the repetitive stimulation The magnitude of the test response (TR) was meas(Fukada, 1971). From these observations, some kind ured as follows. First, the averaged PST-histograms of inhibition is suggested to develop in the neuronal were obtained from 20 consecutive responses to TS network which organizes the RF of Xcell with sucalone, CS alone and to combined stimuli of TS and cessive stimulation, and to spread over the RF center CS, respectively. They were expressed as the number region. An inhibitory mechanism in Y-cell, if any, may of impulses for each bin (bin width was usually 20 be different. msec). The maintained discharge rate averaged for 1 In the present study, the inhibitory processes ’ set prior to the presentation of TS alone was caused by a flashing spot itself in the on-center fields expressed also in impulses/bin. The unconditioned TR of Xand Y-cells were investigated by two-spot experwas defined by a total number of impulses which were iment: Response to the RF-centered spot (test spot, elicited by TS alone and exceeded the maintained TS) was measured when an additional spot (condidischarge rate. When TS was combined with CS. the tioning spot, CS) was presented in the same RF center conditioned TR was obtained in the same way except region. that the maintained discharge rate was substituted by Experiments were conducted on 13 adult cats PST-histograms to CS alone. anaesthetized with nitrous oxide and immobilized Forty on-center cells were classitied as either X (20) with gallamine triethiodide. Extracellular spikes were or Y (20) depending on the two criteria. One criterion recorded from the optic tract with tungsten microelecis the difference in the tonic (X) or phasic (Y) response trodes. Spikes were fed to a Schmitt trigger circuit profile of the cell to a long-lasting spot stimulus. The to produce pulses of standard width and amplitude. other is the difference in the fine structure of the imand those pulses were fed to a mini-computer (TOSpulse train of the initial transient response to the BAC-3CHM, Toshiba) to compile the post-stimulus stimulus onset; Y-cells have a clear discontinuity in time histograms (PST-histograms). their discharge rate between the initial burst and SUCThe TS and CS were produced from glow moduceeding discharges whereas Xcells do not show such lator tubes (Sylvania, R1131Q and those spots of discontinuity (Saito et al.. 1970, 1971). It is worth noting that this difference in the initial transient part of response was so marked that it proved to be a 1 A preliminary report of this work was presented at good criterion for X/Y-classiication. XXVI ICPS Satellite Symposium on Vision held in Sydney. For both types of cells, TR was suppressed by CS October 1974. presentation, but the time course of the suppression

Repetitive firing of the cat's retinal ganglion cell

Abstract The induced activity which was previously found to be elicited by flickering lightspots in Type-I cells (phasic cells) of the cats retina could also be elicited by diffuse flicker and by repetitive antidromic stimulation of the ganglion cell. This activity was elicited when the ganglion cell was activated at high frequency for long period (at 120–250 impulses/sec for about 1 min), and lasted for about 1 min with regularly-spaced high frequency discharges (initial frequency, 100–200 impulses/sec) and suddenly ceased when the discharge rate reduced monotonically to about a half. For Type-II cells (tonic cells), either the flickering light stimulation or the repetitive antidromic stimulation of the cells failed to elicit induced activity . The above differences could reflect the differences in the receptive field organization between Type-I and Type-II cells.

A Spatio-Temporal Learning Rule Based on the Physiological Data of LTP Induction in the Hippocampal CA1 Network

We studied the LTP inducing factors using temporally and spatially modulated stimuli given to the hippocampal neural network. It was found that when the spatial factors were maintained to be constant the positive correlation in the successive inter-stimulus intervals contributes to produce larger LTP. On the other hand, if the temporal factors are kept constant, the spatial coinsidence contributes to poroduce lager LTP. We propose a learning rule by which these experimental results can be consistently interpreted.

Neural substrate for a high degree of noise tolerance in visual flow discrimination in macaque dorsal MST area

To reveal the underlying cellular mechanisms of the noise tolerance in the discrimination of the direction of visual flow, we analyzed both response behaviors of directionally selective cells in the d-MST of the macaque and discrimination properties of human subjects for visual flow by using large-field texture flow. The stimulus was a 32/spl times/32-pixel random checker pattern refreshed at every 16 msec in which coherency of the motion of the texture was systematically controlled by a special purpose computer. The discrimination performance of the human observers showed a high degree of tolerance for directional noise. Field-type D-cells in d-MST also showed response properties which closely resembled human discrimination performance. These results strongly suggest that cells in d-MST area play a crucial role in the generation of signals for perception of global motion.

A HIGH DEGREE OF NOISE TOLERANCE IN HUMAN VISUAL FLOW DISCRIMINATION

We examined the ability of human observers to detect direction of wide-field texture flow when they were shown a moving texture field in which the direction of motion of texture components was not uniform. Stimuli were dynamic random checker patterns a single frame of that was composed of 32×32 square pixels (equal number of light and dark ones) and subtended 43×43 degrees in visual angle. The frame was renewed every 16 msec in a specific fashion such that the percentage of coherency in the direction of positional shift of the light/dark patches to an immidiate neighbor between successive frames was statistically controlled. The human observers were requested to answer the moving direction of the global flow of random checkers. The percentage of correct responses increased rapidly from the chance level near 10% coherency to 90% correct level at 30% coherency. Thus, the human discrimination of the direction of the wide-field texture flow is highly tolerable for the distribution of directional noise in component movements.