Visuo-tactile
cross-modal associations in cortical somatosensory cells
Yong-Di Zhou* and Joaquín M. Fuster
Neuropsychiatric Institute and Brain Research Institute, School of
Medicine, University of California, Los Angeles, CA 90024
Proc Natl Acad Sci U S A. 2000 August 15; 97(17): 9777–9782. PMCID:
PMC16941
Three adult rhesus monkeys (Macaca mulatta) were the experimental
subjects for this study. They had been used in studies of short-term
memory (14, 15). Animal care and surgical procedures were approved by
the Animal Research Committee at the University of California, Los
Angeles. The animals were trained to perform a visuo-haptic memory task
in a fully automated, computer-controlled apparatus. During the task,
the animal was seated in a primate chair facing a panel with a
rectangular screen at about eye level for visual display (30 × 50 mm)
and a rectangular opening at about waist level for access to tactile
test objects (Fig. 1). The distance between the eyes of the animal and
the screen was about 20 cm. A pair of visual images (icons) was used.
These were black and white patterns of parallel stripes (3.5 mm apart).
The stripes were vertical in one icon and horizontal in the other. The
opening for the tactile test objects was normally closed by a shutter.
When the shutter was opened (downward sliding), the animal could reach
out through the opening and manipulate the objects behind the panel (the
objects were at all times out of sight). The test objects were two
vertical cylindrical rods of identical dimensions (axis, 150 mm and
diameter, 19 mm), but different direction of parallel ridges on their
surface (ridges 6 mm apart). One rod had the ridges along the axis of
the cylinder (vertical ridges) and the other around its circumference
(horizontal ridges). When not in the act of reaching and grasping the
objects, the performing hand of the animal rested on a rounded pedal (handrest)
in the center of the lower edge of the opening. The other hand was at
all times restricted from access to the test objects by a plate attached
to the primate chair. A displacement-sensitive transducer was connected
to the spring-suspended seat of the animal. Signals from this transducer
were recorded and used for control of body movements.
A task trial consisted of the following sequence of events—all
registered by electronic switches and sensors. The trial began with the
3-s presentation of one of the icons (vertical or horizontal). The
direction of its stripes symbolized the direction of the ridges in the
rod to be chosen haptically later, after a 20-s delay (14 s in some
tests). At the end of that delay, an auditory signal marked the
accessibility of the two rods side by side. One rod was 5.5 cm to the
right of the center of the opening, and the other 5.5 cm to the left. As
soon as they became accessible, the animal reached out and palpated the
rods. A pull of either rod led to closure of the opening and to fluid
reward (2 ml) if the choice was correct. The icon and the position of
its corresponding rod changed at random between trials. This prevented
the animal from using spatial clues for the choices. Early in the study,
instead of the icon, the monkey was presented with the sight of the
sample object through a window at eye level—later substituted by the
projection display screen.
Before testing the animals on the visuo-haptic task described above,
the animals had been trained on a unimodal (haptic-haptic) task. This
allowed comparisons of unit discharge between the two tasks. In that
unimodal tactile task, a trial started with a click, which served as an
alerting signal about 1.5 s before the touch of the tactile sample. The
sample object was identical to one of the objects used for visuo-haptic
choice. Here, instead of the visual cue, the animal had to palpate that
sample object for the tactile match after the delay. In every other
respect, the task was identical to the cross-modal task.
After the monkey had undergone behavioral training (performance
criterion above 75% correct), two cylindrical pedestals for
microelectrode recording were implanted bilaterally on the parietal
cortex, leaving the dura intact. The pedestals were intended to be
placed over hand representation areas of anterior parietal cortex
(Brodmann's areas 3a, 3b, 1, and 2). The implantation was guided by
cranial landmarks, our own experience with previous implants, and a
stereotaxic map (courtesy of T. P. Pons, Wake Forest University,
Winston-Salem, NC). In one of the animals, a pair of EOG
(electrooculogram) electrodes was implanted in the periorbital bone for
monitoring horizontal eye movements.
For a recording session, the animal was placed in the testing
apparatus with its head fixed. Single-unit activity was recorded
extracellularly with Elgiloy microelectrodes (impedance 1–2 megaohms).
Spike records were selected for analysis on the basis of stability,
uniformity, and clear isolation from background noise and the spikes
from other units.
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