Biomedical subjects

Matteo Carandini

Publications and source records attributed to Matteo Carandini.

4 recordsLinked to original sources

PubMed2002

A synaptic explanation of suppression in visual cortex.

The responses of neurons in the primary visual cortex (V1) are suppressed by mask stimuli that do not elicit responses if presented alone. This suppression is widely believed to be mediated by intracortical inhibition. As an alternative, we propose that it can be explained by thalamocortical synaptic depression. This explanation correctly predicts that suppression is monocular, immune to cortical adaptation, and occurs for mask stimuli that elicit responses in the thalamus but not in the cortex. Depression also explains other phenomena previously ascribed to intracortical inhibition. It explains why responses saturate at high stimulus contrast, whereas selectivity for orientation and spatial frequency is invariant with contrast. It explains why transient responses to flashed stimuli are nonlinear, whereas spatial summation is primarily linear. These results suggest that the very first synapses into the cortex, and not the cortical network, may account for important response properties of V1 neurons.

Action Potentials↗

PubMed2002

Suppression without inhibition in visual cortex.

Neurons in primary visual cortex (V1) are thought to receive inhibition from other V1 neurons selective for a variety of orientations. Evidence for this inhibition is commonly found in cross-orientation suppression: responses of a V1 neuron to optimally oriented bars are suppressed by superimposed mask bars of different orientation. We show, however, that suppression is unlikely to result from intracortical inhibition. First, suppression can be obtained with masks drifting too rapidly to elicit much of a response in cortex. Second, suppression is immune to hyperpolarization (through visual adaptation) of cortical neurons responding to the mask. Signals mediating suppression might originate in thalamus, rather than in cortex. Thalamic neurons exhibit some suppression; additional suppression might arise from depression at thalamocortical synapses. The mechanisms of suppression are subcortical and possibly include the very first synapse into cortex.

Action Potentials↗

PubMed2002

Testing the Bayesian model of perceived speed.

In a recent Bayesian model by Weiss, Simoncelli, and Adelson, motion perception is biased by a prior favoring slow speeds. This model predicts qualitatively an impressive variety of phenomena, including the dependence of perceived speed on contrast. We show that the model can also generate quantitative predictions: for a drifting grating with contrast c, perceived speed is proportional to c(q)/(k(q) + c(q)), with k, q constants. We tested this expression on measurements of perceived speed as a function of contrast. Observers indicated the slower of two drifting gratings, a test and a standard. For each test contrast we found the test speed that appeared to match the standard speed. The model fits the data, but only if q is less than 2, the value it would have if the internal representation of contrast were linear. The Bayesian model can make correct quantitative predictions, but needs to be extended to incorporate a more realistic, nonlinear representation of contrast.

Bayes Theorem↗

PubMed2002

Masking by fast gratings.

Perception of an oriented pattern is impaired in the presence of a superimposed orthogonal mask. This masking effect most likely arises in visual cortex, where neuronal responses are suppressed by masks having a broad range of orientations. Response suppression is commonly ascribed to lateral inhibition between cortical neurons. Recent physiological results, however, have cast doubt on this view: powerful suppression has been observed with masks drifting too rapidly to elicit much of a response in cortex. We show here that the same is true for perceptual masking. From contrast discrimination thresholds, we estimated the cortical response to drifting patterns of various frequencies, and found it greatly reduced above 15-20 Hz. In the same subjects, we measured the strength of masking by the same patterns and found it equally strong for masks drifting slowly (2.7 Hz) as for masks drifting rapidly (27-38 Hz). Fast gratings thus cause strong masking while eliciting weak cortical responses. Our results might be explained by inhibition from cortical neurons that respond to unusually high frequencies, and yet do not make their signals fully available for perceptual judgments. A more parsimonious explanation, however, is that masking does not involve lateral inhibition from cortex. Masking might operate in retina or thalamus, which respond to much higher frequencies than cortex. Masking might also be due to thalamic signals to cortex, perhaps through depression at thalamocortical synapses.

Contrast Sensitivity↗