Elsevier

Neuroscience

Volume 145, Issue 2, 16 March 2007, Pages 775-789
Neuroscience

Systems neuroscience
Effects of perceptual learning in visual backward masking on the responses of macaque inferior temporal neurons

https://doi.org/10.1016/j.neuroscience.2006.12.058Get rights and content

Abstract

Learning is critical for fast and efficient object recognition in primates. To understand the neuronal correlates of behavioral improvements due to training, we recorded the responses of single neurons in the inferior temporal (IT) cortex of monkeys that were trained to recognize briefly presented, backward-masked objects. First we investigated training effects that are specific to the objects shown during training and that do not transfer to untrained objects. Only one of two monkeys tested showed object-specific training effects at the behavioral level, and only this monkey showed a transient object-specific increase in object selectivity for trained compared with untrained backward-masked objects. However, in each monkey a substantial part of the training effect transferred to untrained objects. To investigate the neural correlates of these object-independent training effects, we compared the neural responses to masked objects in trained monkeys to the responses in untrained monkeys. Training was associated with a reduction of the responses to the irrelevant masking patterns. These findings suggest that extensive training in recognizing backward-masked objects results in neural changes that reduce IT responses to the interfering irrelevant masking patterns and enhance the processing of the relevant objects.

Section snippets

Subjects

Four juvenile male monkeys (Macaca mulatta) were used. All procedures were approved by the K. U. Leuven Ethical Committee for animal experiments and followed U.S. National Institutes of Health guidelines. All efforts were made to minimize the number of animals used and their suffering. Separate surgeries were performed in order to implant a plastic head restraint, a scleral search coil, and a plastic recording chamber (Crist Instruments, Hagerstown, MD, USA). Monkeys were anesthetized with

The neural correlates of object-specific learning

The initial aim of our experiments was to investigate the neural correlates of object-specific training effects. Two monkeys (1A and 1B) were trained with a particular set of stimuli, and we compared behavioral performance and neuronal responses after training for this trained set of stimuli and untrained sets of stimuli.

The monkeys performed a masked same/different task in which they compared two objects of which the first was masked (Fig. 1). The effect of SOA was measured for two groups of

Discussion

We found that training in recognizing masked objects improved monkeys’ recognition performance. A substantial part of this training effect transferred to untrained objects. The small object-specific component of training-related improvements, found in only one monkey, was associated with a transient, object-specific increase in neuronal selectivity. A comparison of trained and untrained monkeys suggested that the strong object-independent behavioral effects were associated with a reduction of

Conclusion

In conclusion, our results suggest that training in backward masking is associated with a marked decrease in mask responses, in addition to a transient object-specific increase in object selectivity at short SOA in one monkey. These new results reveal how the visual system deals with the specific constraints of a perceptual learning paradigm, such as backward masking, in order to enhance the processing of degraded but relevant objects and to suppress the processing of salient but irrelevant

Acknowledgment

We thank M. De Paep, P. Kayenbergh, G. Meulemans, and G. Vanparrys for technical assistance, and C. Baker and D. Cox for their helpful comments on a previous version of the manuscript. This work was funded by GSKE and IUAP 5/10 (R.V.), IDO/02/004 (R.V. and J.W.), Human Frontier Science Program (RGP 18/2004 (R.V.)). H.O. was supported by the Human Frontier Science Program and the Fund for Scientific Research Flanders.

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