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.