The receptive field organization of simple cells was studied by analyzing interaction effects between two stationary flashing light stimuli. One stimulus was positioned in the most responsive part of the receptive field to produce activity against which the effects of the other stimulus in various positions of the visual field could be determined.

Receptive field organization of simple cells in cat striate cortex.

The receptive field organization of complex cells was studied by analyzing interaction effects between two stationary flashing light stimuli. One was placed in the most responsive part of the receptive field to produce activity against which effects of the other in different visual field positions could be determined. The receptive field was spatially organized into antagonistic center and flanks just like the fields of simple cells. However, both center and flanks were found within the receptive field area where a single slit evoked discharge. Center and flanks were elongated along the optimal stimulus orientation. The flanks were displaced from the center normal to optimal stimulus orientation. In the center, ON- and OFF-responses were usually about equal in strength and the maximum ON- and OFF-responses occurred in about the same position. This shows that complex cells are activated by input from both ON- and OFF-center cells in the lateral geniculate nucleus (LGN) where the receptive field centers of the LGN cells overlap closely. This explains most of the specific features of complex cells, e.g., the spatially overlapping ON- and OFF-zones, the large response field, the repetitive firing when a slit moves over the receptive field, and the marked non-linear spatial summation. Strong flank suppression occurred with both ON and OFF. The effects were usually stronger on one side of the center. Maximal suppression occurred on the same side with both ON and OFF. This is consistent with the interpretation that complex cells are inhibited by input from both LGN ON- and OFF-center cells with overlapping receptive field centers. A model presuming that complex cells have overlapping but acentric excitatory and inhibitory fields was tested by computer simulation and shown to fit the experimental data. This is the same model as presented for simple cells in the preceding paper (Heggelund 1980), except that the excitatory and inhibitory fields of simple cells have input from either ON- or OFF-center LGN cells, whereas in complex cells they have input from both types.

Receptive field organization of complex cells in cat striate cortex.

Spatial summation was studied quantitatively through width response curves made with an optimally oriented test slit of variable width, and by comparing the response to combined presentation of several parallel slits with the response to each slit alone. Prior to summation analysis, the cell's discharge field (DF) was mapped by presenting a test slit ON and OFF across the receptive field. Activation profiles, showing the extension of subregions where light stimulation increased (enhancement) or decreased the firing rate (suppression), were made by presenting an optimally oriented activation slit in the most responsive DF-position. Against this activity the effects of a parallel test slit were determined in a series of broadside positions. Width response curves were made over the subregions of the DF and the activation profiles. Spatial summation was found in all cells, but the width of the summation region was smaller than the width of the subregions in the respective profiles. The width of the summation region was related to the degree of activation rather than to specific locations within the receptive field. The effect produced by several slits presented together deviated from the algebraic sum of the effects produced by each slit alone. Linear summation was rarely found. Accumulated response curves obtained by integration of DF or activation profiles were compared with width response curves to test linearity of summation. Linear summation throughout the whole receptive field was never found. A satisfactory fit was found only over a narrow region showing that summation was linear within a small part of the summation region. Linearity ended near response maxima or minima in the response profiles. The results indicate that the receptive field of simple cells consists of overlapping excitatory and inhibitory fields, and that the exact location and width of enhancement and suppression zones are determined by an activity-dependent balance between excitatory and inhibitory inputs.

Spatial summation in the receptive field of simple cells in the cat striate cortex

1. Single unit responses of pan-directional cells to moving and stationary flashing stimuli were studied in the superficial layers of the superior colliculus in paralysed, anaesthetized rhesus monkeys. The aim of this study was to see how far cell responses to moving stimuli fit in with what would be expected from their responses to stationary flashing stimuli. 2. Both the leading and the trailing edge of a moving stimulus evoke a transient response. If the diameter of moving light spots is increased the strength of the leading edge response increases, reaches a maximum and decreases to a constant value which is similar to the behaviour of the on response when the diameter of flashing spots is increased. The strength of the trailing edge response increases and reaches the same strength as that of the leading edge response. If the width of a long moving slit is increased, the strength of the leading edge response is the same at all slit widths, while the strength of the trailing edge response shows a course similar to that of the trailing edge response if the spot diameter is increased. If the length of a wide moving slit is increased both the leading and the trailing edge responses decrease. These results indicate that the strength of both leading and trailing edge responses is dependent on the degree the inhibitory surround is activated. 3. The leading and the trailing edge of a stimulus evoke their responses at the same position in the receptive field independent of the direction of movement. 4. Increasing the velocity of a moving stimulus shows that in general the leading edge response is present up to higher velocities than the trailing edge response independent of the sign of contrast. The burst duration to moving stimuli decreases with increasing stimulus velocity and appears to be determined by the time a moving edge is present in the receptive field centre. When this time becomes shorter than 10--20 ms, the burst duration for moving stimuli is constant and about the same as for flashing stimuli. This indicates that, although spatial receptive field properties can vary considerably, temporal receptive field properties show a strong similarity among different units. 5. The response latencies to light and dark moving edges are the same, which in turn are about equal to the response latencies to stationary flashing stimuli. 6. Stimulation experiments show that the general response characteristics to moving stimuli can be predicted by using a set of receptive field parameters derived from responses to stationary flashing stimuli. The most important variable of moving stimuli appears to be the period of time a moving contour is present within the receptive field centre, besides the degree of activation of the inhibitory surround.

Responses of single units in the monkey superior colliculus to moving stimuli.

The relationship between orientation selectivity and spatial receptive field organization was analyzed Receptive field maps were made with a dual stimulus technique where an optimally oriented activation slit was presented in the most responsive region to produce activity against which the effect of a test spot in various positions was determined Both simple and complex cells had receptive fields which were subdivided into adjacent elongated and antagonistic subrogions When the two stimuli were presented in phase (both ON or OFF simultaneously) the fields had a central enhancement region with a strong suppression flank on one or both sides Optimal slit orientation was related to the location of the suppression flank relative to the location of the central enhancement region, and the degree of orientation selectivity to the shape of the subregions and the distance between them Estimated orientation tuning curves calculated from the receptive field maps gave satisfactory first approximations to experimental curves

Orientation selectivity and the spatial distribution of enhancement and suppression in receptive fields of cat striate cortex cells.

The spatial resolution of simple cells in cat visual cortex was measured by stimulation with pairs of 6? wide parallel light bars of various spacings. These double lines were moved across the receptive field and were taken as resolved if there was a 10% deflection between the double peak responses of cells. As a control, recordings were also made from several geniculate fibers. The smallest bar separations resolved by simple cells were larger than those which have been found for cells of the lateral geniculate nucleus (LGN), although the smallest cortical receptive field centers were as small as those of LGN-cells. The correlation between optimal resolving power of a cell and the width of its excitatory receptive field was much weaker in cortical simple cells than in LGN cells. In contrast to the LGN, the double line responses of most simple cells differ markedly from an additive superposition of two single line responses spaced according to the actual interline distance. As possible mechanisms underlying these nonlinearities three different connectivity schemes were investigated. Two of these models were based on receptive field concepts; the third one used intracortical circuits. Only the latter model could explain all the nonlinear effects seen in the neurophysiological experiments.

H. C. Nothdurft

Papers

Spatial resolution and nonlinearities of simple cells in the cat visual cortex measured with parallel line pairs