Transcript SFN07ARC

TEMPORAL DYNAMICS OF SPATIAL ATTENTION IN V1 AND V4 OF AWAKE, BEHAVING MONKEYS
1,2
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Jitendra Sharma* , Hiroki Sugihara , James Schummers , and Mriganka Sur
527.1
1Picower
Institute for Learning and Memory, Department of Brain and Cognitive Sciences, MIT, Cambridge, MA 02139
2Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA 02129
Introduction
Results
Background
1.
Visuo-spatial attention involves a two way process of enhancing perception of the attended stimulus while inhibiting
non-attended stimuli. Attentional modulations have been likened to contrast gain mechanisms that serve to bias
competitive interactions between multiple stimuli towards the attended stimulus. However, the locus and mechanism
underlying such biased processing remains unresolved. Furthermore, most previous studies on attention have shown
that there occurs a simple increase in neuronal firing rate to the attended stimulus. It however remains unclear if
particular characteristics of stimulus such as size, orientation, contrast or spatial frequency of engage greater or less
attention and whether such a change dynamically modify spatial and temporal dimensions of neuronal responses. In
this study we simultaneously recorded neuronal responses in V1 and V4 in awake behaving monkeys to a dynamically
changing stimulus pattern that overlapped receptive fields (RF) of single neurons in both areas. The monkeys were
trained in a covert spatial attention task where monkeys passively attended towards or away from the RF continuously
for the entire period of the trial. We used a reverse correlation paradigm to study the dynamics of attentional modulation
for different stimulus dimensions.
3. Effects of surround stimulation on orientation dynamics
Attention modulates firing rate response to dynamic grating stimuli
V4 (n=71) Monkeys AB & AU
V4
V1 (n=129) Monkey AB
Attention spot location L1
Normalized
Response
Towards
-3
x 10
Methods:
Small
Attention
Index
Behavioral Task: Monkeys were trained to covertly attend to a small attention spot that could appear towards the RF
on the recorded neurons or away from it, in a contralateral location. The task sequence started with monkey holding a
lever that brought a red fixation spot (0.1 deg.). They were required to hold fixation within a small window such that any
jitter in fixation > 0.25 deg. aborted the trial. Once the monkey acquired stable fixation, an attention spot and an
equiluminant distracter were simultaneously presented towards the RF or in the contralateral hemifield. Monkeys were
required to hold fixation release the lever within 350-400ms of the disappearance of the attention spot. The attention
spot could appear towards the RF (attend Towards) or away from the RF (attend Away) in a random order. To prevent
monkeys from anticipating attention spot disappearance, the trials could last in time windows of 600ms, 1200 ms and
1800 ms, and within each window there was a further randomization of up to 500 ms. This made the task attentionally
demanding.
Away
-3
x 10
10
Large
Small
Large
8
6
4
2
0
--- Attend Away
Attention spot location L2
Normalized
Response
Reward
7
6
5
4
3
2
1
0
-1
-2
-2
--- Attend Towards
Behavioral Control & Recording: The behavioral task and stimulus presentation was controlled by CORTEX (NIMH
freeware). Extracellular unit recordings were done in V1 and V4 from neurons having receptive fields centers within 6°
from the center of gaze. Before commencing the recording, RF size, location and other characteristics were carefully
mapped to optimize stimulus location. Spike sorting and post processing were done using custom software written in
Matlab.
V1
Population average tuning curve vs. time plots for V4 neurons (left) and V1 neurons (right). The top row contains
tuning curves for the attend toward condition, and the bottom row for attend away. Tuning curves were calculated
independently from stimuli that were small (close to RF size) and large (3X RF size).
Response
(Bar Release)
400 ms
t3= t2+500ms
(randomized)
Stimulus on
Stimulus
Stimulus
V4
t2= t1+ 600/1200/1800ms
(randomized)
Distractor
t1= t+300ms
Top Traces (in color): Population average PSTH representation of responses to the dynamic grating stimulus for V4 neurons
(left) and V1 neurons (right), under two behavioral conditions: Attention towards the RF, or Attention away from the RF. Attention
was directed at one of two randomly chosen locations (L1 & L2) outside the RF. Bottom Traces (in black): Time course of the
Attention Index of the firing rate ([FRtow-FRawy]/[FRtow+FRawy]) during the stimulus presentation.
Fixation
Fix-spot
Time (msec)
Time (msec)
L1
Attention
spot
V1
.8
L2
Attention Cue
Time course of surround suppression in V1 and V4
Time = t
Normalized
Probability
CRF
4.
Attention
Index
Attention cue
off
0
0
20 40 60 80 100 120 140 160 180 200
Time (msec.)
0
20 40 60 80 100 120 140 160 180 200
Time (msec.)
0
20 40 60 80 100 120 140 160 180 200
Time (msec.)
0
20 40 60 80 100 120 140 160 180 200
Time (msec.)
Bar Press
2.
Attention Towards
Population average dynamic tuning curves
Attention Away
RF
Small
Large
Time
Stimuli: The stimulus consisted of patches of gray
sinusoidal gratings consisting of 8 orientations, two
different sizes (2° and 5° for V1 neurons and 4°
and 8° for V4 neurons) and two contrasts of 20%
and 60%. Spatial frequencies were optimized for V1
and V4 neurons (typically 3 cycles /deg and 0.5
cycles / deg. respectively). Each stimulus frame was
presented in rapid sequence at the monitor refresh
rate (75 Hz) and lasted from 1100-2300 ms. There
were a total of 360 conditions that were presented in
a pseudo-random order. Identical patterns of stimuli
were presented covering the RF and in the
contralateral location. The attention spot and
distracter locations were randomly chosen at two
points just outside the RF so as to obviate any
anticipation bias for a particular location around the
RF.
Data Analysis: Responses to individual frames in
the stimulus were assessed with the reversecorrelation technique. For each time delay (tau)
from 0-200msec, the probability distribution of
stimulus frames presented “tau” msec before a
spike was calculated. The response to the blank
stimulus frames was subtracted from the response
to each stimulus. Orientation tuning curves were
then calculated for each “tau” under four conditions
consisting of two attention states: towards or away
from the RF, and stimulus size: small or large.
V4
Spike Train
V1
Population average response dynamics. Each plot shows the average time course of response across cells to the
preferred orientation. V4 neurons show positive-going responses for small stimulus size, but the response is
almost entirely suppressive for large stimuli. In V1 neurons, the large stimuli result in larger, shorter responses.
Towards
t
Summary
Stimulus
V1 awy sm late
-3
~13 msec
Early
Away
x 10
0.035
12
0.03
10
0.025
8
0.02
6
4
Late
Early
0.015
0.01
2
0.005
0
0
Late
-2
…
1) Spatial attention to the receptive field location leads to increased coarse firing
rate in response to dynamic grating stimulus movies
2) Neurons in V4 showed mild to moderate bias for location of attention spot
placed around the RF.
3) Time course of attention modulation in V4 neurons showed two distinct
components, an early and a late component. In contrast, V1 neurons showed
only late component.
…
…
Population average tuning curve vs. time plots for V4 neurons (left) and V1 neurons (right). The top row contains
tuning curves for the attend toward condition, and the bottom row for attend away. Tuning curves were calculated
independently from stimuli in the early portion of the trial (first 300 msec) and the late portion of the trial (300 to 2100
msec).
4) While orientation tuning in subset of neurons was affected by the attentional
state, averaged over entire population, we did not find robust effect of
attention on orientation tuning in either V1 or V4.
5) Neurons in V4 showed strong Iso-oriented surround suppression that does
not get affected by the spatial attention.