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{\LARGE Visual Attention:
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Lab Report for Experiment \#1}
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{\Large (9.63 Laboratory in Cognitive Science)}
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{\large Juliet C. Bourne
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April 7, 1992}
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\begin{quotation}
{\footnotesize
In an experiment modeled after those described by Michael Posner in his
1980 paper, {\em Orienting Attention}, the effect that shifting visual
attention (independently of eye movements) has on the detection of a
stimulus is explored. The costs and benefits of having a cue (which can
be valid, invalid, or neutral) to prompt visual attention in a given
direction (or no direction at all) are measured over three different
delays (from cue to stimulus). The results seem to indicate a definite
effect for both the various cue types and the various delay values.
However, the shape of this effect is strange, with the costs and
benefits varying from one delay value to another. This could be
attributed to the ``time lock'' of the shifting of attention. It appears
that sometime after 200ms but before 500ms, visual attention shifts to
the target area; after 500ms but before 800ms visual attention is no
longer at the target area.}
\end{quotation}
\vspace*{.4in}

As established by Michael Posner (and others), shifting visual attention
can be shown to be independent of overt movements of the head or eyes
toward the target. In this experiment, this result is duplicated with an
added twist hinted at by Posner. In addition to the benefits and costs,
in terms of reaction times, of orienting visual attention based on a cue
which might or might not be right, the effects of the delay from the
time the cue is given to the time the stimulus appears are also studied.

The task is a simple one: pressing a key in response to an asterisk
which appears on the screen, while fixating on the middle of the screen.
Cues are given as to which side of the fixation point the asterisk is
likely to appear. Since this task requires no visual acuity, it is
assumed that subjects keep their eyes on the fixation point while
shifting their visual attention to a periphery position. 

The benefits and costs can be measured using this simple task. When the
cue is accurate (and thus the subjects have moved their attention to the
correct side of fixation), reaction times should be faster than when the
cue is neutral (i.e. the subjects presumably haven't moved their
attention from the fixation point). Also, when the cue is wrong
(subjects have their visual attention on the wrong side of the fixation
point), reaction times should be slower than when the cue is neutral.
The delay condition should help to show the ``time lock'' of shifting
visual attention. That is, how long does it take to shift visual
attention from fixation to target and how long does it remain there? If
the benefits and costs are different for the different delay values,
then rough conclusions can be made about the timing of shifting visual
attention.

\newpage
\sect{Method}

\subsect{Subjects}

The subjects for this experiment consisted of sixteen MIT students,
mainly undergraduates, enrolled in the Laboratory for Cognitive Science.

\subsect{Procedure}

Subjects were told that their task would be to press the \verb+z+ key on
the computer keyboard as soon as they saw an asterisk (\verb+*+) appear
on the computer screen in front of them. They were asked to fixate their
eyes on, and not move them from, the space between the two \verb-+-
signs that would be in the middle of the screen. They were then informed
that a cue (\verb+<+, \verb+>+, or \verb+=+) would appear between the
two \verb-+- signs. The cue \verb+<+ meant that there was a 65-70\% chance
that the asterisk would appear to the left of the fixation point. A
\verb+>+ meant that there was a 65-70\% chance the asterisk would appear on
the right, while a \verb+=+ meant that there was an equal chance that the
asterisk would appear on either side of the fixation point. Subjects
were also told that on some trials the asterisk would not appear.

The trials were broken into several blocks. Subjects were told that
they should rest only at the end of a block if they needed to. They were
also asked to alternate hands after each block.

\subsect{Design}

The design of this experiment involves two independent variables, delay
and cue validity. Delay is the amount of time after the cue is displayed
until the asterisk appears. There are three delay values: 200ms, 500ms,
and 800ms. Cue validity is whether the cue displayed accurately
reflected where the asterisk appeared. The three values for cue validity
are: valid (correct), neutral, and invalid (false). A valid cue means
that the asterisk appeared where the cue said it would. Neutral cue
validity means that the cue displayed was \verb+=+ and thus the
subjects' visual attention would not be drawn in one direction or
another. An invalid cue means that the asterisk appeared opposite where
the cue said it would.

Each subject saw 60 valid cue trials, 36 invalid cue trials, and 37
\footnote{For some strange reason, there is an extra 800ms neutral trial.}
neutral trials. There are 44 trials with a 200ms delay, 44 with a 500ms
delay, and 45\footnotemark{1} with a 800ms delay. Thus, there are 20
valid trials, 12 invalid, and 12\footnotemark{1} neutral trials for each
delay value. \setcounter{footnote}{1} In addition, there are 31
``catch'' trials in which an asterisk never appears and 6 practice
trials \footnote{Subjects were not told that there were any practice
trials.} which appear at the beginning and are not used as data. The
non-practice trials were suitably randomized. \footnote{I am assuming
that there were different versions, with different ordering of trials.
But I didn't design this experiment and we weren't told this
explicitly.}

\sect{Results}

The results show a statistically significant ($p<.025$) effect for the
cue validity variable. From this it can be concluded that, within 2.5\%
probability of being a false effect, the cue validity (valid, neutral,
invalid) had an effect on reaction time. A statistically significant
($p<.01$) effect for the delay variable was also found.  Within 1\% of
being a false effect, the delay from the time the cue was presented to
the time the stimulus appeared had an effect on reaction time. The
interaction between the delay and the cue validity was also found to be
statistically significant ($p<.01$).  Thus, within 1\% of being falsely
significant, the reaction time for a given delay value varied with cue
validity.

The attentional cost (invalid cue reaction time minus neutral cue
reaction time) overall was only approximately 5ms, whereas the
attentional benefit (neutral cue reaction time minus valid cue reaction
time) was approximately 13ms (see figure 1). \begin{figure}
\epsfbox{cue.ps} \caption{Mean subject reaction time for each cue
type: valid (stimulus appears where cue said it would), neutral (no
direction specified by cue), and invalid (stimulus appears opposite
where cue said it would).} \end{figure} However, the attentional costs
and benefits vary greatly between each delay condition (see figure 2).
\begin{figure} \epsfbox{all.ps} \caption{Mean subject reaction time for
each cue type, broken into the three delay (time between cue and
stimulus) values.} \end{figure} In the 200ms delay condition, the
attentional cost was approximately -11ms (i.e. it appeared to be a
benefit). 

Overall, the reaction times for valid and invalid cue trials were less
than those for neutral cue trials. In the 800ms delay condition, the
attentional benefit was approximately -20ms (i.e.  it appeared to be a
cost). The reaction times for valid and invalid cue trials were, on
average, greater than those for neutral cue trials. Only in the 500ms
delay condition, the benefit and cost appeared roughly as expected.

As an example of the variability of costs and benefits among subjects,
consider one subject's data. In figure 3, notice that the overall
benefit is approximately 100ms. \begin{figure} \epsfbox{cue-me.ps}
\caption{One subject's mean reaction time for each cue type.}
\end{figure} However, the cost is approximately -50ms. Thus, the overall
reaction time differs by only 50ms for valid cue and invalid cue trials,
with neutral cue trials having the slowest reaction time. In figure 4,
the data are broken into their respective delay conditions.
\begin{figure} \epsfbox{all-me.ps} \caption{One subject's mean reaction
time for each cue type, broken into the three delay values.}
\end{figure} Both the 200ms and 500ms delay conditions show the same
general shape as the 200ms delay condition for all subjects. In this
case, however, the benefits and (negative) costs are roughly equal.
Only the 800ms delay condition shows the expected benefit and cost
behavior.

\sect{Discussion}
The results show that the cue validity has a definite effect on reaction
times. As in Posner (1980) there are costs and benefits associated with
shifting visual attention. When compared to neutral cue trials, reaction
times in valid cue trials are faster and those in invalid cue trials are
slower. However, when the data are broken into their respective delay
conditions, very different costs and benefits are seen. Only at a delay
of 500ms do the cost and benefit of shifting visual attention follow the
expected pattern, with a rather large benefit and a small cost. 

The fact that results at the three delay points vary seems to indicate
that visual attention is not instant nor long-lasting. The effect in the
200ms delay condition is odd, since both valid and invalid cue trials
have reaction times smaller than the neutral cue trials. However, it is
not hard to see that visual attention has not been completely shifted;
the reaction times for all cue validities are longer than in the other
two delay conditions. The effect in the 800ms delay condition is almost
the opposite of that of the 200ms delay. Reaction times in the valid
and invalid cue trials are roughly equal to each other, but slower than
in the neutral cue trials. At this point, visual attention no longer
seems to be focused on the target. Thus, it seems to take between 200ms
and 500ms to shift visual attention to a point other than fixation and
visual attention on the target seems to fade away after between 500ms
and 800ms. 

However, individual differences should not be ignored. Looking at the
one subject's data presented previously, the same general pattern, with
a different ``time lock'' for shifting visual attention, can be
observed. In this case, the effect for delays of 200ms and 500ms are
similar to the overall 200ms delay pattern. The effect at 800ms of
delay looks much like that of the overall 500ms effect, that is there is
a clear benefit and cost. Thus it would seem that for this subject,
shifting visual attention from fixation to target takes between 500ms
and 800ms. Since no reaction times were measured for delays past 800ms,
it can not be determined how long visual attention remains on the target
before drifting for this subject. However, the overall pattern seems to
hold.\footnote{Each subject's data should be looked at in a similar
fashion to see if it shows the same general pattern.}

Thus, with this simple experiment, two phenomena were shown: visual
attention effects stimulus detection and shifting visual attention takes
time and is not long-lived. 

\newpage
\sect{References}

Posner, Michael I. ``Orienting of Attention,'' {\it Quarterly Journal of
Experimental Psychology} (1980) 32, 3-25.

\end{document}

