1. Differential Impact of Aging and Alzheimer’s on Cortical Visual Processing
Cormac A. Assink*, Colin T. Lockwood, and Charles J. Duffy
Dept. of Neurology, Univ. of Rochester Med. Ctr., Rochester, NY 14642
* UofR Summer Scholar
INTRODUCTION
BEHAVIOR SUMMARY: CONCLUSIONS:
This work was supported by grants from NEI R01EY022062 and ONR N000141110525.
ATTENTIONAL NEUROPHYSIOLOGY OF AGING AND AD AGING AND AD EFFECTS ON RESPONSE COHERENCE
AGING AND AD EFFECTS ON RESPONSE POWERMETHODS: Exogenous Pre-Cued Optic Flow Heading Discrimination
1) Subject fixates centered target followed by random dot motion
2) Flashed pre-cue 20o to left or right of fixation:
75% valid = same side as optic flow; 25% invalid = opposite side
3) 400 or 700 ms SOA to onset of left or right heading radial optic flow
4) Left or right button press reporting perceived heading direction.
Valid Invalid
YN
ON
AD
Time (ms) Time (ms)
FrequencyFrequencyFrequency
Valid Invalid
YN
ON
AD
Time (ms) Time (ms)
FrequencyFrequencyFrequency
We recorded optic flow responses in
55 subjects: 18 young normal controls
(YN), 19 older normal controls (ON),
and 18 patients with early AD (AD).
Neuropsychological testing included a
battery of nine instruments with
results that were consistent with
group assignment.
ERPs were recorded with a 10-20 cap
system and analyzed using EEGlab
and ERPlab in Matlab.
Performance declines were seen in
aging and AD, especially for AD in
trials with a short SOA.
We measured inter-trial coherence
(ITC) of valid and invalid responses
at a central electrode, CPZ.
YNs showed strong ITC at the N200
and P300 intervals, with a notch of
incoherence at the time of the N2b.
ONs showed increased N200 ITC
and decreased P300 ITC.
ADs showed decreased N200 ITC
and further decreased P300 ITC.
We measured event-related spectral
perturbations (ERSP) of valid and
invalid responses, also at CPZ.
YNs showed greater power in invalid
trials at the time of the N2b.
ONs show increased power at the
N200 interval, with decreased power at
higher frequencies in the P300 interval.
ADs show diminished ERSPs affecting
power increases in the low frequencies
and power decreases in the high
frequencies.
The radial patterns of visual motion in optic
flow inform us about our self-movement. A
brief stimulus can distract us from our heading
direction and cause potentially fatal driving
errors. Older adults and Alzheimer’s patients
are particularly susceptible to such errors.
We presented optic flow stimuli in a left vs.
right heading discrimination task. The heading
stimuli were preceded by a valid pre-cue,
directing attention to the correct side, or an
invalid pre-cue, directing attention to the other
side. We recorded optic flow event related
potentials (ERPs) from the scalps of human
subjects to assess the impact of pre-cues.
Short SOA Long SOAValid pre-cue
Invalid pre-cue
-5
0
5
10
0 200 400 600
-5
0
5
10
0 200 400 600
0 200 400 600
Time (ms) Time (ms)
CPZ
μV
AD
ON
YN
CPZ
μV
AD
ON
YN
N200
P300
N2b
CPZ
μV
0 200 400 600
0 200 400 600
0 200 400 600
-5
0
5
10
-5
0
5
10
-5
0
5
10
-5
0
5
10
An N200 component is seen in the ERPs of all three subject groups, with short
(400 ms) and long (700 ms) SOAs, and with valid and invalid pre-cueing. The
N2b component is best seen as the difference between valid and invalid ERPs
in YNs, is present but less evident in ONs, and not clearly discernible in ADs.
A P300 is largest in YNs, and is successively diminished in ONs and ADs.
Topographic head-plots for ERPs show ipsi/contra differences between valid
and invalid trials for the N200, N2b, and P300s. YNs show an ipsilateral N2b
in the short SOA, which switches to the contralateral side in the long SOA.
This pattern is also seen in the ONs, although the response is later than in
YNs. In AD, there are no clear lateralized differences between valid and
invalid with short SOAs, but small contralateral effects with long SOAs.
Pre-cueing optic flow has behavioral
and physiological effects that
distinguish aging and AD:
1) N200 amplitude increases in
aging and decreases in AD
2) N2b distinguishes valid and
invalid pre-cues in YNs and ONs
3) P300s are larger in YNs’ invalid
trials, decreased in ONs and ADs
4) ITCs show an N2b notch in YNs,
a loss of P300 ITC in ONs and a
loss of N200 ITC in ADs
5) ERSPs show reciprocal power
changes in ONs and a loss of
power in ADs
In YNs and ONs, the N2b is associated
with attentional cueing, shifting over
time from the ipsi- to the contra-lateral
hemisphere. In ADs, attentional effects
on behavior, and N2b responses are
lost, especially with short SOAs.
Aging is associated with reciprocal
decreases in coherence and increases
in power output, whereas AD shows a
collapse of both coherence and power.
ERPs reveal fundamental differences
between aging and AD, suggesting a
potential pathophysiological cascade
from asynchrony to unresponsiveness.
Fixation Random Pre-cue Random Optic Flow Response
Ipsi
Valid minus
Invalid
N200 N2b P300 N200 N2b P300
N200 N2b P300 N200 N2b P300
N200 N2b P300 N200 N2b P300
Contra
0
100
200
300
400
500
600
700
YN ON AD
Valid Short
Invalid Short
Valid Long
Invalid Long
0
20
40
60
YN ON AD
Short SOA
Long SOA
Button Press Response Times
Validity Effect (Valid RT – Invalid RT)
Time(ms)Time(ms)
Valid minus
Invalid
Valid minus
Invalid