Laboratory of Neurobiology

Torsten Wiesel
Professor Emeritus

Judith A. Hirsch

Each successive stage of cortical processing changes the neural representation of visual space. Our research aims to reveal how the cortical circuitry gives rise to these transformations. One avenue of approach is making whole-cell recordings with dye filled electrodes in vivo (with José-Manuel Alonso and R. Clay Reid). With this technique we can resolve intracellular events during natural stimulation and correlate the physiological pattern of response with anatomical cell class. One ongoing study focuses on the first level of cortical integration, that is, on the cells that thalamic fibers contact. This work reveals the importance of active linear and nonlinear mechanisms in enhancing sensitivity to the position of oriented contours, a basic cortical response property. Recently, we examined the role of dendritic action potentials in translating incoming synaptic input. We found that regenerative calcium potentials enhance excitatory synaptic drive at all stages of cortical processing.

With tissue slices maintained in vitro, we study the synaptic physiology of specific components of the columnar microcircuit. The current emphasis is on understanding the circuitry that generates suppressive response properties, such as length tuning or the failure of diffuse, unpatterned light to evoke firing. We record postsynaptic responses of intracellularly labeled cells to activation of separate, identified, presynaptic pathways. This work reveals distinct classes of inhibitory neurons and we now examine the means by which they incorporate thalamic input with that from intracortical sources.


Jose-Manuel Alonso

Neurons within the visual cortex show a great variety of functional receptive field properties. This functional diversity emerges mainly from interactions between thalamic and corticocortical inputs whose precise circuitry is largely unknown. In our research, we use two different approaches to study this circuitry and the origin of cortical receptive fields. First, we record simultaneously from cells providing cortical and/or thalamic inputs and their target neurons in the cortex. By using this approach, we can measure different types of connectivity and study the receptive field properties of both inputs and targets. Second, we block reversibly restricted regions of the thalamus by making small GABA injections. This approach allows us to study the contribution of separate thalamocortical inputs to the generation of cortical receptive fields.

We found that the direct connections from thalamus to layer IV neurons in cortex are very precise and follow certain rules making it possible, from receptive field analysis, to predict if a thalamic and a layer IV neuron are connected. In addition, we discovered that the direct connections from thalamus to layer IV neurons are reinforced by a tight one millisecond synchrony of the thalamic inputs. The thalamic inputs that arrive simultaneously to the same layer IV neuron are more effective in producing a postsynaptic response than non-simultaneous inputs.

These findings strongly support an original model that proposed convergent thalamic inputs generate cortical receptive fields in layer IV. Our future goal is to study a next step in cortical processing: the role of layer IV inputs in the generation layer II-III complex receptive fields.


Publications Publications