For more details on how we estimated information in neural responses, see
I. Nelken, G. Chechik, T.D. Mrsic Flogel A.J. King and
J.W.H. Schupp
Encoding stimulus information by spike numbers and mean
response time in primary auditory cortex.
J. Computational Neuroscience 19(2):199-221, (2005)
Neurons can transmit information about sensory stimuli via their
firing rate, spike latency, or by the occurrence of complex spike
patterns. Identifying which aspects of the neural responses actually
encode sensory information remains a fundamental question in
neuroscience. Here we compared various approaches for estimating the
information transmitted by neurons in auditory cortex in two very
different experimental paradigms, one measuring spatial tuning and the
other responses to complex natural stimuli. We demonstrate that, in
both cases, spike counts and mean response times jointly carry
essentially all the available information about the stimuli. Thus, in
auditory cortex, whereas spike counts carry only partial information
about stimulus identity or location, the additional availability of
relatively coarse temporal information is sufficient in order to
extract essentially all the sensory information available in the spike
discharge pattern, at least for the relatively short stimuli (< 100
ms) commonly used in auditory research.
For a discussion of the paper, see
Jan Schnupp
Auditory Filters, Features, and Redundant Representations.
Neuron, Vol 51, 278-280, (2006)
Responses in auditory cortex tend to be weaker, more phasic, and
noisier than those of auditory brainstem and midbrain nuclei. Is the
activity in cortex therefore merely a "degraded echo" of lower-level
neural representations? In this issue of Neuron, Chechik and
colleagues show that, while cortical responses indeed convey less
sensory information than auditory midbrain neurons, their responses
are also much less redundant.
For more details on the auditory responses to the same stimuli, see
Omer Bar-Yosef, Yaron Rotman and Israel Nelken
Responses of neurons in cat primary auditory cortex to bird
chirps: effects of temporal and spectral context.
J. Neurosci. 22, 8619-8632 (2002)
The responses of neurons to natural sounds and simplified
natural sounds were recorded in the primary auditory cortex (AI) of
halothane-anesthetized cats. Bird chirps were used as the base natural
stimuli. They were first presented within the original acoustic
context (at least 250 msec of sounds before and after each chirp). The
first simplification step consisted of extracting a short segment
containing just the chirp from the longer segment. For the second
step, the chirp was cleaned of its accompanying background
noise. Finally, each chirp was replaced by an artificial version that
had approximately the same frequency trajectory but with constant
amplitude. Neurons had a wide range of different response patterns to
these stimuli, and many neurons had late response components in
addition, or instead of, their onset responses. In general, every
simplification step had a substantial influence on the
responses. Neither the extracted chirp nor the clean chirp evoked a
similar response to the chirp presented within its acoustic
context. The extracted chirp evoked different responses than its clean
version. The artificial chirps evoked stronger responses with a
shorter latency than the corresponding clean chirp because of envelope
differences. These results illustrate the sensitivity of neurons in AI
to small perturbations of their acoustic input. In particular, they
pose a challenge to models based on linear summation of energy within
a spectrotemporal receptive field.