Biomedical subjects

Luis M Martinez

Publications and source records attributed to Luis M Martinez.

4 recordsLinked to original sources

PubMed2003

Functionally distinct inhibitory neurons at the first stage of visual cortical processing.

Here we explore inhibitory circuits at the thalamocortical stage of processing in layer 4 of the cat's visual cortex, focusing on the anatomy and physiology of the interneurons themselves. Our immediate aim was to explore the inhibitory mechanisms that contribute to orientation selectivity, perhaps the most dramatic response property to emerge across the thalamocortical synapse. The broader goal was to understand how inhibitory circuits operate. Using whole-cell recording in cats in vivo, we found that layer 4 contains two populations of inhibitory cells defined by receptive field class--simple and complex. The simple cells were selective for stimulus orientation, whereas the complex cells were not. Our observations help to explain how neurons become sensitive to stimulus orientation and maintain that selectivity as stimulus contrast changes. Overall, the work suggests that different sources of inhibition, either selective for specific features or broadly tuned, interact to provide appropriate representations of elements within the environment.

Animals↗

PubMed2003

Complex receptive fields in primary visual cortex.

In the early 1960s, Hubel and Wiesel reported the first physiological description of cells in cat primary visual cortex. They distinguished two main cell types: simple cells and complex cells. Based on their distinct response properties, they suggested that the two cell types could represent two consecutive stages in receptive-field construction. Since the 1960s, new experimental and computational evidence provided serious alternatives to this hierarchical model. Parallel models put forward the idea that both simple and complex receptive fields could be built in parallel by direct geniculate inputs. Recurrent models suggested that simple cells and complex cells may not be different cell types after all. To this day, a consensus among hierarchical, parallel, and recurrent models has been difficult to attain; however, the circuitry used by all models is becoming increasingly similar. The authors review theoretical and experimental evidence for each line of models emphasizing their strengths and weaknesses.

Animals↗

PubMed2002

Laminar processing of stimulus orientation in cat visual cortex.

One of the most salient features to emerge in visual cortex is sensitivity to stimulus orientation. Here we asked if orientation selectivity, once established, is altered by successive stages of cortical processing. We measured patterns of orientation selectivity at all depths of the cat's visual cortex by making whole-cell recordings with dye-filled electrodes. Our results show that the synaptic representation of orientation indeed changes with position in the microcircuit, as information passes from layer 4 to layer 2+3 to layer 5. At the earliest cortical stage, for simple cells in layer 4, orientation tuning curves for excitation (depolarization) and inhibition (hyperpolarization) had similar peaks (within 0-7 deg, n = 11) and bandwidths. Further, the sharpness of orientation selectivity covaried with receptive field geometry (r = 0.74) - the more elongated the strongest subregion, the shaper the tuning. Tuning curves for complex cells in layer 2+3 also had similar peaks (within 0-4 deg, n = 7) and bandwidths. By contrast, at a later station, layer 5, the preferred orientation for excitation and inhibition diverged such that the peaks of the tuning curves could be as far as 90 deg apart (average separation, 54 deg; n = 6). Our results support the growing consensus that orientation selectivity is generated at the earliest cortical level and structured similarly for excitation and inhibition. Moreover, our novel finding that the relative tuning of excitation and inhibition changes with laminar position helps resolve prior controversy about orientation selectivity at later phases of processing and gives a mechanistic view of how the cortical circuitry recodes orientation.

Action Potentials↗

PubMed2002

Synaptic physiology of the flow of information in the cat's visual cortex in vivo.

Each stage of the striate cortical circuit extracts novel information about the visual environment. We asked if this analytic process reflected laminar variations in synaptic physiology by making whole-cell recording with dye-filled electrodes from the cat's visual cortex and thalamus; the stimuli were flashed spots. Thalamic afferents terminate in layer 4, which contains two types of cell, simple and complex, distinguished by the spatial structure of the receptive field. Previously, we had found that the postsynaptic and spike responses of simple cells reliably followed the time course of flash-evoked thalamic activity. Here we report that complex cells in layer 4 (or cells intermediate between simple and complex) similarly reprised thalamic activity (response/trial, 99 +/- 1.9 %; response duration 159 +/- 57 ms; latency 25 +/- 4 ms; average +/- standard deviation; n = 7). Thus, all cells in layer 4 share a common synaptic physiology that allows secure integration of thalamic input. By contrast, at the second cortical stage (layer 2+3), where layer 4 directs its output, postsynaptic responses did not track simple patterns of antecedent activity. Typical responses to the static stimulus were intermittent and brief (response/trial, 31 +/- 40 %; response duration 72 +/- 60 ms, latency 39 +/- 7 ms; n = 11). Only richer stimuli like those including motion evoked reliable responses. All told, the second level of cortical processing differs markedly from the first. At that later stage, ascending information seems strongly gated by connections between cortical neurons. Inputs must be combined in newly specified patterns to influence intracortical stages of processing.

Action Potentials↗