Resumen:
Experimental findings show the ubiquitous presence of graded responses and tuning
curves in the neocortex, particularly in visual areas [1-15]. Among these,
inferotemporal-cortex (IT) neurons respond to complex visual stimuli, but
differences in the neurons' responses can be used to distinguish the stimuli
eliciting the responses [8, 9, 16-18]. The IT projects directly to the medial
temporal lobe (MTL) [19], where neurons respond selectively to different pictures of
specific persons and even to their written and spoken names [20-22]. However, it is
not clear whether this is done through a graded coding, as in the neocortex, or a
truly invariant code, in which the response-eliciting stimuli cannot be
distinguished from each other. To address this issue, we recorded single neurons
during the repeated presentation of different stimuli (pictures and written and
spoken names) corresponding to the same persons. Using statistical tests and a
decoding approach, we found that only in a minority of cases can the different
pictures of a given person be distinguished from the neurons' responses and that in
a larger proportion of cases, the responses to the pictures were different to the
ones to the written and spoken names. We argue that MTL neurons tend to lack a
representation of sensory features (particularly within a sensory modality), which
can be advantageous for the memory function attributed to this area [23-25], and
that a full representation of memories is given by a combination of mostly invariant
coding in the MTL with a representation of sensory features in the neocortex.
