A simple single-unit microelectrode recording system

A simplified method for manufacturing glass-insulated metal microelectrodes

A simplified method to manufacture durable, glass-insulated, tungsten microelectrodes with sufficient control of the final electrode impedance is described. This method requires only two instruments, an electrolytic etcher for wires and pipette puller, for manufacturing these electrodes. The manufacture of these electrodes involves 3 steps: (1) etching tungsten wire to sharpen the tip, (2) insulating the electrode by pulling a glass pipette over the sharpened tungsten wire and (3) assessing and adjusting the tip exposure and impedance of the electrode to meet recording requirements. Control over the electrode impedance is easily accomplished by varying the distance between the uppermost portion of the heating coil and the sharpened wire tip before a glass pipette is pulled over the wire tip. This distance determines the area of tip exposure and also the location where the glass insulation ends and the exposed electrode tip begins. A performance test of these electrodes in a chronically prepared monkey showed that they were strong enough to repeatedly penetrate thickened dura mater without significant changes in impedance and to isolate cortical neuronal activity after these multiple penetrations. Furthermore, the strength of these microelectrodes eliminated the need to remove reactive granular tissue from the dura overlying the recording site.

Multiplexing in single cells of the alert monkeys visual cortex during brightness discrimination

Recordings were made from neurons in parafoveal striate cortex while monkey performed a brightness discrimination task. The earliest post-stimulus firing reflected only stimulus parameters. Later firing as 44% greater on trials where the monkeys performed correctly. A third epoch of cell activity, averaged backward in time from the response, showed a different temporal pattern preceding correct responses than preceding mistakes. The results show concomitants of behavioral plasticity in primary sensory cortex.

Temporal response characteristics of cells in monkey striate cortex measured with metacontrast masking and brightness discrimination

After measuring metacontrast masking psychophysically in two monkeys, recordings from parafoveal striate cortex of the monkeys were made while they performed a simultaneous brightness discrimination and while they judged the apparent brightness of a stimulus masked by metacontrast. The size and orientation of the stimuli were held constant regardless of receptive field parameters. In both tasks, the single-cell activity immediately following the presentation of flashed discrimination stimuli reflected only stimulus parameters, and was independent of the monkey's behavioral choice. Later activity (up to 400 msec post-stimulus) was significantly greater if the monkey was about to press the correct panel in the discrimination, or if he pressed the unmasked side (with greater apparent brightness but identical intensity) in the masking paradigm. One quarter of the cells showed a change in firing rate during the 250 msec preceding the behavioral response, though the difference in overall firing level between correct and incorrect brightness discrimination trials was diminished in this epoch, and the corresponding firing difference in metacontrast trials was not significant. The temporal pattern of firing also differed between correct and incorrect trials in the pre-response interval. The results suggest an iterative or recurrent coding of visual information, where the same cells participate in early, late, preresponse coding in different ways.

Plasticity: the mirror of experience

A simple avoidance training procedure during early development produces massive neural traces in visual and somatic cortices of kittens reared in a normal environment. A preponderance of cells in these areas had response preferences for the stimuli used during training. Furthermore, some of these cells exhibited properties never found in normal animals not receiving such training. It appears that, even in an environment in which many other stimuli are present, some early experiences powerfully affect brain development and the way in which other experiences exert their effect.

Visual receptive fields sensitive to absolute and relative motion during tracking

Some neurons in the visual cortex of awake monkeys visually tracking a moving target showed receptive fields that were excited only by stimulus motion relative to a background, while other neurons responded to any kind of stimulus motion. This result was found with two methods, one in which tracking eye movements were identical in both relative-motion and absolute-motion conditions, and another in which stimulus motions on the retina were identical in both conditions. This response pattern can differentiate translation of the retinal image during eye movement from motion of objects in the world.