Responses to pulses and sinusoids in macaque ganglion cells

Suppression of the magnocellular pathway during saccades

Saccades create two problems for the visual system: they cause fast (but resolvable) motion of the retinal image and a change in the relationship between retinal and external spatial co-ordinates. In this review, we examine the first of these problems, of why there is no distributing sense of motion during saccades. Recent evidence from a range of sources suggests that during saccades, the magnocellular pathway is selectively suppressed, while the parvocellular pathway is functionally unimpaired, or even enhanced. The suppression seems to occur early, possibly in the lateral geniculate nucleus, where the pathways are well separated. It is possible that the suppression shares similar mechanisms to those responsible for contrast gain control.

Temporal impulse response functions for luminance and colour during saccades

Previous work has shown that during saccadic eye movements, contrast sensitivity for low spatial frequency patterns modulated in luminance is selectively reduced by up to one logarithmic unit, while high spatial frequency patterns, and equiluminant patterns of all spatial frequencies are not suppressed at all [Burr et al. (1994). Nature, 371, 511-513]. Here we study the temporal characteristics for sensitivity to luminance and chromatic patterns during saccades, using the two-pulse summation technique. Sensitivity was measured for detecting two successive pulses as a function of stimulus-onset asynchrony, during normal viewing and during saccades. Impulse response functions were estimated from the summation data, for all conditions. For equiluminance, the functions were monophasic during normal viewing and saccades. For luminance modulation, the impulse response functions were di-phasic in both normal viewing and saccades. However, during saccades the impulse responses were faster in normal viewing. This result is consistent with the suggestion that saccadic suppression is mediated by contrast gain control mechanisms, known to occur in M-cells but not P-cells.

Receptive field structure in the primate retina

This review summarizes recent work relevant to receptive field structure of cells of the parvocellular (PC) and (MC) magnocellular pathways in the primate. In the PC-pathway, recent data suggest that different color- and cone-opponent ganglion cells make up specific anatomical classes with specific cone connectivities and bipolar cell input. For example, blue-on ganglion cells have been identified anatomically as the small bistratified ganglion cell class. For the midget ganglion cells, which appear to be red-green opponent, there seems to be only one mosaic for red and green on-center and one for red and green off-center cells. This mixture of cell type within a retinal cell mosaic is unusual, as is the fact that dendritic trees of neighboring midget cells do not overlap. Physiologically, all PC-cells lack a contrast gain control mechanism and show a high degree of spatial and temporal linearity of their responses. In the magnocellular pathway, on- and off-center cells, corresponding to parasol cells with dendritic trees ramifying in the inner and outer sublaminae of the inner plexiform layer, show properties familiar from studies of cat ganglion cells, e.g. a contrast gain control is present. However, a chromatic input to the receptive field surround gives their responses an additional order of complexity.

The spatial precision of macaque ganglion cell responses in relation to vernier acuity of human observers

Responses of parafoveal macaque ganglion cells were measured as a function of the contrast and position of an edge flashed within their receptive fields. The goal was to determine the ability of different cell types to signal edge location. For comparison, parafoveal vernier thresholds of human observers were measured with pairs of flashed edges. Cells of the magnocellular (MC-) pathway gave larger responses than cells of the parvocellular (PC-) pathway. Neurometric analyses comparing a cell's response at different edge positions were performed. The positional signal from single MC-pathway cells was more precise than from PC-pathway cells, especially at lower contrasts. In a second analysis, based on the neurophysiological results, responses from a matrix of ganglion cells were generated. Using a simple model, vernier performance expected from such a matrix was predicted as a function of edge length and contrast. Again, the MC-pathway gave a more precise positional signal than the PC-pathway despite the latter's numerical advantage. At contrasts of 20% and below, only the MC-pathway would appear capable of supporting vernier performance with our stimuli. At higher contrasts either the MC- or PC-pathway could provide an adequate signal.

Dependence of visual latency on wavelength: predictions of a neural counting model

Simple reaction time (RT) was determined as a function of wavelength for equally visible, near-threshold stimuli. The stimuli were 5-deg spectral onsets of 1,000-ms duration presented on a 100-Td spatially coincident white background. All three subjects manifested the same result: RTs were fastest in the region of 590 nm. These data were analyzed in the context of a counting model of visual latency. This model predicts that for equally visible stimuli a transient detector will result in shorter visual latencies than will a more sustained detector. On the basis of this analysis, it is concluded that although most long duration, near-threshold, spectral step onsets are detected by the sustained parvocellular pathway, an exception occurs in the region of 590 nm: these stimuli are detected by a relatively transient pathway, presumably the magnocellular pathway.

Temporal response of ganglion cells of the macaque retina to cone-specific modulation

The temporal response of cone inputs to macaque retinal ganglion cells were compared with cone-specific sinusoidal modulation used to isolate each cone type. For all cell types of the parvocellular (PC) pathway, temporal responsivity was similar for short (S)-, middle (M)-, and long (L)-wavelength-sensitive cone inputs, apart from small latency differences between inputs to center and surround. The temporal response resembled that expected from receptor physiology. Responses of cells of the magnocellular pathway to M- or L-cone modulation showed more complex properties indicative of postreceptoral processing. Human psychophysical temporal-sensitivity functions were acquired with S-cone modulation under conditions similar to those for the physiological measurements. Ratios of psychophysical to physiological data from S-cone cells (the only cells that respond to this stimulus) yielded an estimate of the central filter acting upon PC-pathway signals. The filter characteristic could be described by a four-stage low-pass filter with corner frequency 3-5 Hz.