eyes in the back of the head
Every kid knows that moms have “eyes in the back of their heads.” We are adept at fixing our gaze on one object while independently directing attention to others. Salk Institute neurobiologists are beginning to tease apart the complex brain networks that enable humans and other higher mammals to achieve this feat.
Each round of the game began when the eyes fixated a white dot surrounded by four striped circular stimuli on a video screen. A camera followed and recorded pupil position throughout the session, enabling the researchers to track eye position, which is indicated by small red cross. Two of the striped stimuli flashed momentarily, marking them as targets that should be attended, while continuing to stare at the central spot. The circle shows the location of the receptive field of the neuron under study on the day that this demonstration was recorded. Neuronal responses from individual rounds of the game are illustrated. Tick marks indicate when the neuron emitted individual action potentials. The researchers compared neuronal responses when the stimulus in the receptive field was tracked (red tick marks) or ignored (blue tick marks). The effect of attention differed markedly across the two different types of neurons. (Credit: Image courtesy of Salk Institute)
Researchers report two classes of brain cells with distinct roles in visual attention, and highlight at least two mechanisms by which these cells mediate attention. “This study represents a major advance in our understanding of visual cognition, because it is the first study of attention to distinguish between different classes of neurons,” says system neurobiologist John Reynolds, Ph.D., associate professor in the Systems Neurobiology Laboratory at the Salk Institute.
In the experiments, animals learned how to play a sophisticated video game, which challenged their visual attention-focusing skills. During the game, the Salk researchers recorded electrical activity from individual neurons in part of the visual cortex that has been implicated in mediating visual attention.
As illustrated in the demonstration (see link below), the neurons respond when a stimulus appears within a window (indicated by the circle) covering a small part of the visual field that the eye sees. This window is known as the neuron’s “receptive field.” Whenever the stimuli entered the neuron’s receptive field, the cell produced a volley of electrical spikes, known as “action potentials”, indicated by vertical tick marks in the demonstration.
On some trials, attention was directed to the stimulus that entered the neuron’s receptive field, while on other trials attention was instead directed to the other stimuli. The researchers recorded almost 200 different neurons, and examined how each neuron’s response changed when attention was directed to the stimulus in its receptive field.
They found that neurons typically responded more strongly when attention was directed to the stimulus in their receptive fields. Upon closer inspection, however, the researchers noticed that different neurons produced different shaped electrical spikes: “broad spikes” and “narrow spikes.” Other researchers had previously identified two different types of neurons that produce these two waveforms.
The most common neuron type, called a pyramidal cell, produces broad spikes. These neurons transmit signals between different brain areas. The other class, fast-spike interneurons evoke narrow spikes. These neurons only connect to their local neighboring neurons, and are involved in local computations.
After sorting the neurons by waveform, the researchers observed that attention had different effects on the two different types of neurons. The narrow-spiking cells typically fired more frequently when the tracked object was attended than when it was unattended. Broad-spiking cells, on the other hand, were less influenced by attention. Some fired faster, while others fired more slowly when attention was directed to the stimulus in the receptive field. What’s more, attention caused the stream of spikes produced by the narrow-spiking neurons to be much more reliable.
“By distinguishing among the different neural elements that make up the cortical circuit, we are gaining a view of the biological underpinnings of attention that is unprecedented in its level of detail,” says post-doctoral researcher Jude Mitchell, Ph.D., lead author on the study. He adds that, while there is much more work ahead, “if we can understand how attention is acting on different cell classes, this will significantly improve our understanding of the pathology of neurological diseases in which attention is impaired.”
Note: The video can be viewed at http://www.snl.salk.edu/~jude/attentiontask.avi
Neuron. 2007 Jul 5;55(1):131-41.
Differential Attention-Dependent Response Modulation across Cell Classes in Macaque Visual Area V4.
Mitchell JF, Sundberg KA, Reynolds JH – Systems Neurobiology Lab, The Salk Institute, La Jolla, CA 92037-1099, USA.
The cortex contains multiple cell types, but studies of attention have not distinguished between them, limiting understanding of the local circuits that transform attentional feedback into improved visual processing. Parvalbumin-expressing inhibitory interneurons can be distinguished from pyramidal neurons based on their briefer action potential durations. We recorded neurons in area V4 as monkeys performed an attention-demanding task. We find that the distribution of action potential durations is strongly bimodal. Neurons with narrow action potentials have higher firing rates and larger attention-dependent increases in absolute firing rate than neurons with broad action potentials. The percentage increase in response is similar across the two classes. We also find evidence that attention increases the reliability of the neuronal response. This modulation is more than two-fold stronger among putative interneurons. These findings lead to the surprising conclusion that the strongest attentional modulation occurs among local interneurons that do not transmit signals between areas.
PMID: 17610822 [PubMed – in process]