Orientation in visual cortex: a simple mechanism emerges.

The experiments reported by Ferster et al. 1996xFerster, D, Chung, S, and Wheat, H. Nature, in press. 1996; See all ReferencesFerster et al. 1996 challenge the notion that intracortical networks of any form, excitatory or inhibitory, are required for generating orientation selectivity. They measure orientation responses of layer 4 simple cells from normal cat cortex with intracellular patch electrodes. They then cool the cortex to the point where cortical neurons in all layers through layer 4 can no longer generate output spikes. This effectively quenches any intracortical interactions, except for a highly attenuated contribution from layer 6, which the authors argue is too weak to be significant. Thus, in the cooled cortex, layer 4 simple cell intracellular voltages almost exclusively reflect LGN input. Yet the visually evoked synaptic potentials in these cells, reduced many fold by cooling, remain sharply orientation tuned in a manner displayed by the cell before cooling. These experiments do not address the question of whether orientation is created de novo in other cortical layers or simply transmitted vertically from layer 4, but the results confirm one major tenet of Hubel and Wiesel's original model of orientation selectivity: simple cells in layer 4 require only LGN input to define their orientation selectivity.The importance of these results extends beyond the issue of understanding how visual cortex acquires orientation. Orientation selectivity is not the only neuronal property that emerges for the first time in V1; most neurons in cat V1 are also sharply tuned for the direction of motion of moving stimuli, whereas LGN afferents are not. Mechanisms proposed to explain directionality have evolved through many of the same stages that models for orientation selectivity have (e.g.,Suarez et al. 1995xSuarez, H.H, Koch, C, and Douglas, R. J. Neurosci. 1995; 15: 6700–6719PubMedSee all ReferencesSuarez et al. 1995). In their current paper, Ferster and his colleagues provide some evidence that the directionality of a cortical neuron is also defined through afferent inputs alone. These results open up the possibility that a similar principle may apply at higher levels of cortex; the basic features of the more complicated receptive fields at higher levels could also be defined by patterns of afferent input alone. This brings us, finally, to a very interesting question to which their paper leads: what might be the role of the rather substantial intracortical excitatory and inhibitory input to layer 4 cells, if silencing it leaves the defining features of a cell unchanged? One suggestion is that these intrinsic connections correct for the nonlinearities of layer 4 cells. This could explain how V1 cells maintain orientation (and direction) selectivity notably unchanged over a wide range of stimulus intensities and contrasts (Carandini and Heeger 1994xCarandini, M and Heeger, D.J. Science. 1994; 264: 1333–1336Crossref | PubMedSee all ReferencesCarandini and Heeger 1994). These are the sorts of questions that should now be asked in our attempt to understand the basic mechanisms of cortical visual processing.