Tuesday, May 15, 2007

Behavioral Detection of Electrical Microstimulation in Different Cortical Visual Areas

Dona K. Murphey, John H.R. Maunsell
Current Biology 17, 862–867, May 15, 2007

The extent to which areas in the visual cerebral cortex
differ in their ability to support perceptions has been
the subject of considerable speculation. Experiments
examining the activity of individual neurons have suggested
that activity in later stages of the visual cortex
is more closely linked to perception than that in earlier
stages [1–9]. In contrast, results from functional imaging,
transcranial magnetic stimulation, and lesion
studies have been interpreted as showing that earlier
stages are more closely coupled to perception [10–
15]. We examined whether neuronal activity in early
and later stages differs in its ability to support detectable
signals by measuring behavioral thresholds for
detecting electrical microstimulation in different cortical
areas in two monkeys. By training the animals to
perform a two-alternative temporal forced-choice
task, we obtained criterion-free thresholds from five
visual areas—V1, V2, V3A, MT, and the inferotemporal
cortex. Every site tested yielded a reliable threshold.
Thresholds varied little within and between visual
areas, rising gradually from early to later stages. We
similarly found no systematic differences in the slopes
of the psychometric detection functions from different
areas. These results suggest that neuronal signals of
similar magnitude evoked in any part of visual cortex
can generate percepts.

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