Neurons in striate cortex limit the spatial and temporal resolution for detecting disparity modulation.

Stereopsis is the process of seeing depth constructed from binocular disparity. The human ability to perceive modulation of disparity over space (Tyler, 1974; Prince and Rogers, 1998; Banks et al., 2004a) and time (Norcia and Tyler, 1984) is surprisingly poor, compared with the ability to detect spatial and temporal modulation of luminance contrast. In order to examine the physiological basis of this poor spatial and temporal resolution of stereopsis, I quantified responses to disparity modulation in disparity selective V1 neurons from four awake behaving monkeys. To study the physiological basis of the spatial resolution of stereopsis, I characterized the three-dimensional structure of 55 V1 receptive fields (RF) using random dot stereograms in which disparity varied as a sinusoidal function of vertical position (“corrugations”). At low spatial frequencies, this produced a modulation in neuronal firing at the temporal frequency of the stimulus. As the spatial frequency increased, the modulation reduced. The mean response rate changed little, and was close to that produced by a uniform stimulus at the mean disparity of the corrugation. In 48/55 (91%) of the neurons, the modulation strength was a lowpass function of spatial frequency. These results suggest that the neurons have fronto-parallel planar receptive fields, no disparity-based surround inhibition and no selectivity for disparity gradients. This scheme predicts a relationship between RF size and the high frequency cutoff. Comparison with independent measurements of RF size was compatible with this. All of this behavior closely matches the binocular energy model, which functionally corresponds to cross-correlation: the disparity modulated activity of the binocular neuron measures the correlation between the filtered monocular images. To examine the physiological basis of the temporal resolution of stereopsis, I measured for 59 neurons the temporal frequency tuning with random dot stereograms in which disparity varied as a sinusoidal function of time. Temporal frequency tuning in response to disparity modulation was not correlated with temporal frequency tuning in response to contrast modulation, and had lower temporal frequency high cutoffs on average. The temporal frequency high cut for disparity modulation was negatively correlated with the response latency, the speed of the response onset and the temporal integration time (slope of the line relating response phase and temporal frequency). Binocular cross-correlation of the monocular images after bandpass filtering can explain all these results. Average peak temporal frequency in response to disparity modulation was 2Hz, similar to the values I found in four human observers (1.5-3Hz). The mean cutoff spatial frequency, 0.5 cpd, was similar to equivalent measures of decline in human psychophysical sensitivity for such depth corrugations as a function of frequency (Tyler, 1974; Prince and Rogers, 1998; Banks et al., 2004a). This suggests that the human temporal and spatial resolution for stereopsis is limited by selectivity of V1 neurons. For both, space and time, the lower resolution for disparity modulation than for contrast modulation can be explained by a single mechanism, binocular cross-correlation of the monocular images. The findings also represent a significant step towards understanding the process by which neurons solve the stereo correspondence problem (Julesz, 1971)

Decision-Related Activity in Sensory Neurons May Depend on the Columnar Architecture of Cerebral Cortex

Many studies have reported correlations between the activity of sensory neurons and animals' judgments in discrimination tasks. Here, we suggest that such neuron-behavior correlations may require a cortical map for the task relevant features. This would explain why studies using discrimination tasks based on disparity in area V1 have not found these correlations: V1 contains no map for disparity. This scheme predicts that activity of V1 neurons correlates with decisions in an orientation-discrimination task. To test this prediction, we trained two macaque monkeys in a coarse orientation discrimination task using band-pass-filtered dynamic noise. The two orientations were always 90° apart and task difficulty was controlled by varying the orientation bandwidth of the filter. While the trained animals performed this task, we recorded from orientation-selective V1 neurons (n = 82, n = 31 for Monkey 1, n = 51 for Monkey 2). For both monkeys, we observed significant correlation (quantified as “choice probabilities”) of the V1 activity with the monkeys' perceptual judgments (mean choice probability 0.54, p = 10(−5)). In one of these animals, we had previously measured choice probabilities in a disparity discrimination task in V1, which had been at chance (0.49, not significantly different from 0.5). The choice probabilities in this monkey for the orientation discrimination task were significantly larger than those for the disparity discrimination task (p = 0.032). These results are predicted by our suggestion that choice probabilities are only observed for cortical sensory neurons that are organized in maps for the task-relevant feature

Monoaminergic Neuromodulation of Sensory Processing

All neuronal circuits are subject to neuromodulation. Modulatory effects on neuronal processing and resulting behavioral changes are most commonly reported for higher order cognitive brain functions. Comparatively little is known about how neuromodulators shape processing in sensory brain areas that provide the signals for downstream regions to operate on. In this article, we review the current knowledge about how the monoamine neuromodulators serotonin, dopamine and noradrenaline influence the representation of sensory stimuli in the mammalian sensory system. We review the functional organization of the monoaminergic brainstem neuromodulatory systems in relation to their role for sensory processing and summarize recent neurophysiological evidence showing that monoamines have diverse effects on early sensory processing, including changes in gain and in the precision of neuronal responses to sensory inputs. We also highlight the substantial evidence for complementarity between these neuromodulatory systems with different patterns of innervation across brain areas and cortical layers as well as distinct neuromodulatory actions. Studying the effects of neuromodulators at various target sites is a crucial step in the development of a mechanistic understanding of neuronal information processing in the healthy brain and in the generation and maintenance of mental diseases