Spatial and temporal response properties of the major retino-geniculate pathways of Old and New World monkeys

Evaluating spatiotemporal interactions between shapes

Spatiotemporal interactions between stimuli can alter the perceived curvature along the outline of a shape (Habak, Wilkinson, Zakher, & Wilson, 2004; Habak, Wilkinson, & Wilson, 2006). To better understand these interactions, we used a forward and backward masking paradigm with radial frequency (RF) contours while measuring RF detection thresholds. In Experiment 1, we presented a mask alongside a target contour and altered the stimulus onset asynchrony between this target-mask pair and a temporal mask. We found that a temporal mask increased thresholds when it preceded the target-mask stimulus by 130-180 ms but decreased thresholds when it followed the target-stimulus mask by 180 ms. Furthermore, Experiment 2 demonstrated that the effects of temporal and spatial masks are approximately additive. We discuss these findings in relation to theories of transient and sustained channels in vision.

Spatial receptive field properties of lateral geniculate cells in the owl monkey (Aotus azarae) at different contrasts: a comparative study

Several physiological properties of owl monkey lateral geniculate nucleus (LGN) cells were studied to verify whether its nocturnal habit has an influence on the organization of its subcortical visual system. Receptive field (RF) dimensions were measured using drifting gratings and bipartite field stimuli. We found that owl monkey cells LGN have larger RFs and were on average more non-linear than those of diurnal monkeys. But, as in other anthropoids, there is an increase in RF centre size with increasing eccentricity, and there is a limited correlation between these centre sizes and retinal ganglion cell dendritic tree sizes. The influence of contrast on sizes and peak sensitivities of RF centres and surrounds and on the response phases was studied. Both the sizes and peak sensitivities of the RF centres and surrounds decrease as contrast increases. As a result, the responses to low spatial frequency stimuli saturate with increasing contrast. Estimates of contrasts at half-maximal responses confirm the presence of saturation. It was found that the magnocellular cells saturate more strongly than parvocellular cells. The response phase increases with increasing contrast. These data resemble those obtained in the common marmoset, indicating that these are basic features of the primate visual system. We conclude that during evolution and as an adaptation to a nocturnal lifestyle, cells in the owl monkey LGN display an increased spatial integration in comparison with diurnal primate species, without a change in the basic organization common to the primate subcortical visual system.

Spatial and temporal frequency selectivity of neurons in the middle temporal visual area of new world monkeys (Callithrix jacchus)

Information about the responses of neurons to the spatial and temporal frequencies of visual stimuli is important for understanding the types of computations being performed in different visual areas. We characterized the spatiotemporal selectivity of neurons in the middle temporal area (MT), which is deemed central for the processing of direction and speed of motion. Recordings obtained in marmoset monkeys using high-contrast sine-wave gratings as stimuli revealed that the majority of neurons had bandpass spatial and temporal frequency tuning, and that the selectivity for these parameters was largely separable. Only in about one-third of the cells was inseparable spatiotemporal tuning detected, this typically being in the form of an increase in the optimal temporal frequency as a function of increasing grating spatial frequency. However, most of these interactions were weak, and only 10% of neurons showed spatial frequency-invariant representation of speed. Cells with inseparable spatiotemporal tuning were most commonly found in the infragranular layers, raising the possibility that they form part of the feedback from MT to caudal visual areas. While spatial frequency tuning curves were approximately scale-invariant on a logarithmic scale, temporal frequency tuning curves covering different portions of the spectrum showed marked and systematic changes. Thus, MT neurons can be reasonably described as similarly built spatial frequency filters, each covering a different dynamic range. The small proportion of speed-tuned neurons, together with the laminar position of these units, are compatible with the idea that an explicit neural representation of speed emerges from computations performed in MT.

Spatial frequency-specific potentiation of human visual-evoked potentials

Recent research has suggested that cortical long-term potentiation can be induced non-invasively in humans by using rapid visual stimulation. The present study extends these findings by investigating the specificity of this long-term potentiation effect to the inducing stimulus. One group of study participants were tetanized using a one cycle-per-degree sine grating, while a second group was tetanized using a five cycles-per-degree sine grating. Using electroencephalography, we found that an increase in the N1b potential was specific to sine gratings of the same frequency as the tetanus. No effect was observed in the N1b for sine gratings of a different spatial frequency. These results suggest that the long-term potentiation effect induced by the sensory tetanus is isolated to a discrete neural population in the visual cortex.

Morphology and physiology of primate M- and P-cells

Catarrhines and platyrrhines, the so-called Old- and New-World anthropoids, have different cone photopigments. Postreceptoral mechanisms must have co-evolved with the receptors to provide trichromatic color vision, and so it is important to compare postreceptoral processes in these two primate groups, both from anatomical and physiological perspectives. The morphology of ganglion cells has been studied in the retina of catarrhines such as the diurnal and trichromatic Macaca, as well as platyrrhines such as the diurnal, di- or trichromatic Cebus, and the nocturnal, monochromatic Aotus. Diurnal platyrrhines, both di- and trichromats, have ganglion cell classes very similar to those found in catarrhines: M (parasol), P (midget), small-field bistratified, and several classes of wide-field ganglion cells. In the fovea of all diurnal anthropoids, P-cell dendritic trees contact single midget bipolars, which contact single cones. The Aotus retina has far fewer cones than diurnal species, but M- and P-cells are similar to those in diurnal primates although of larger size. As in diurnal anthropoids, in the Aotus, the majority of midget bipolar cells, found in the central 2 mm of eccentricity, receive input from a single cone and the sizes of their axon terminals match the sizes of P-cell dendritic fields in the same region. The visual responses of retinal ganglion cells of these species have been studied using single-unit electrophysiological recordings. Recordings from retinal ganglion cells in Cebus and Aotus showed that they have very similar properties as those in the macaque, except that P-cells of mono- and dichromatic animals lack cone opponency. Whatever the original role of the M- and P-cells was, they are likely to have evolved prior to the divergence of catarrhines and platyrrhines. M- and P-cell systems thus appear to be strongly conserved in the various primate species. The reasons for this may lie in the roles of these systems for both achromatic and chromatic vision.

The what and where in visual masking

A metacontrast mask suppresses the visibility of, without influencing the reaction time (RT) to, the target. We investigated whether this dissociation results from a sensori-motor pathway immune to masking effects or from the characteristics of stimulus timing in mutually inhibitory sustained and transient channels. For target visibility, para- and metacontrast yielded the usual U-shaped functions. Peak paracontrast occurred at stimulus onset asynchronies (SOAs) of -150 to -100 ms. RTs were relatively low for metacontrast and did not show a systematic change as a function of SOA. The RT contribution from contour-masking was greatest at an SOA of -150 ms (paracontrast) and declined to near zero in the metacontrast regime. The dissociation between visibility and RT seen in metacontrast did not occur in paracontrast, rejecting the theory that RTs are elicited by a single sensori-motor pathway immune to masking. The dependence of the dissociation on stimulus timing can be explained by RECOD, a dual-pathway model wherein fast and slow activities interact.

The mammalian photoreceptor mosaic-adaptive design

Unlike in birds and cold-blooded vertebrates' retinas, the photoreceptors of mammalian retinas were long supposed to be morphologically uniform and difficult to distinguish into subtypes. A number of new techniques have now begun to overcome the previous limitations. A hitherto unexpected variability of spectral and morphological subtypes and topographic patterns of distribution in the various retinas are being revealed. We begin to understand the design of the photoreceptor mosaics, the constraints of evolutionary history and the ecological specialization of these mosaics in all the mammalian subgroups. The review discusses current cytological identification of mammalian photoreceptor types and speculates on the likely "bottleneck-scenario" for the origin of the basic design of the mammalian retina. It then provides a brief synopsis of current data on the photoreceptors in the various mammalian orders and derives some trends for phenomena such as rod/cone dualism, spectral range, preservation or loss of double cones and oil droplets, photopigment co-expression and mono- and tri-chromacy. Finally, we attempt to demonstrate that, building on the limits of an ancient rod dominant (probably dichromatic) model, mammalian retinas have developed considerable radiation. Comparing the nonprimate models with the intensively studied primate model should provide us with a deeper understanding of the basic design of the mammalian retina.