Browsing by Author "Fallah, Mazyar"

Abstract Background Feature-based attention prioritizes the processing of the attended feature while strongly suppressing the processing of nearby ones. This creates a non-linearity or “attentional suppressive surround” predicted by the Selective Tuning model of visual attention. However, previously reported effects of feature-based attention on neuronal responses are linear, e.g., feature-similarity gain. Here, we investigated this apparent contradiction by neurophysiological and psychophysical approaches. Results Responses of motion direction-selective neurons in area MT/MST of monkeys were recorded during a motion task. When attention was allocated to a stimulus moving in the neurons’ preferred direction, response tuning curves showed its minimum for directions 60–90° away from the preferred direction, an attentional suppressive surround. This effect was modeled via the interaction of two Gaussian fields representing excitatory narrowly tuned and inhibitory widely tuned inputs into a neuron, with feature-based attention predominantly increasing the gain of inhibitory inputs. We further showed using a motion repulsion paradigm in humans that feature-based attention produces a similar non-linearity on motion discrimination performance. Conclusions Our results link the gain modulation of neuronal inputs and tuning curves examined through the feature-similarity gain lens to the attentional impact on neural population responses predicted by the Selective Tuning model, providing a unified framework for the documented effects of feature-based attention on neuronal responses and behavior.

The production of new hippocampal neurons in adulthood has been well documented in rodents. Recent studies have extended these findings to other mammalian species, such as tree shrews and marmoset monkeys. However, hippocampal neurogenesis has not been demonstrated in adult Old World primates. To investigate this possibility, we injected 11 adult Old World monkeys of different ages (5–23 years) with the thymidine analog bromodeoxyuridine and examined the fate of the labeled cells at different survival times by using neuronal and glial markers. In the young-adult and middle-aged monkeys, we found a substantial number of cells that incorporated bromodeoxyuridine and exhibited morphological and biochemical characteristics of immature and mature neurons. New cells located in the dentate gyrus expressed a marker of immature granule neurons, Turned On After Division 64 kDa protein, as well as markers of mature granule neurons including neuron specific enolase, neuronal nuclei, and the calcium-binding protein calbindin. Fewer new cells expressed the astroglial marker glial fibrillary acidic protein. Evidence of neurogenesis was observed in the oldest monkeys (23 years) as well, but it appeared to be less robust. These results indicate that the adult brains of Old World monkeys produce new hippocampal neurons. Adult macaque monkeys may provide a useful primate model for studying the functional significance of adult neurogenesis.