Experimental Psychology PSY 433 Chapter 8 Attention and Reaction Time

Where’s Waldo? http://www.youtube.com/watch?v=EvWh6PMi9Ek&feature=player_embedded#

Two Aspects of Attention Divided attention – what happens when we try to engage in two cognitive processes at once Selective attention – how we switch mental resources from one cognitive task to another.

Visible Bottleneck Task http://opl.apa.org/contributions/Pashler/prp.html This task illustrates how difficult it is to pay attention to two things at the same time. Both tasks require a choice of response and the same cognitive resource cannot be devoted to both tasks at the same time. The competition goes away when one task does not involve a choice (e.g., press any button when you hear a tone).

Donders A, B & C Revisited A is simple RT – see a stimulus and press a key B is stimulus choice RT – see one of two stimuli and decide whether to press a key or not C is stimulus and response choice RT – see one of two stimuli and decide which of two keys to press B-C gives response selection time C-A gives ID time.

Donders Tasks S1  R1 Donders A S1  R1 Donders B S2  R2 S1  R1 Donders C S2

Donders A -- Simple A Reaction Time C Reaction Time C Minus A Baseline Identification Time Selection Time

Donders B -- Choice B Reaction Time C Reaction Time B Minus C

Testing for Modularity Donders A, B & C implies that the parts of the task are modules. How can component modules be identified? When one component module can be changed without changing the others, it is independent. If Donders was correct then the three parts should be separately modifiable. Pure insertion – addition of a module without affecting the duration of the other modules.

Response Force Donders pure insertion could not be tested so an additional variable was added – response force. Response force – the amount of pressure exerted on a response key. Force increases with stimulus intensity. For Ulrich et al. (1999), response force was the same for a Donders A and B comparison, even though RTs were different.

RT and Integrated Force (Fig 8.3) This result is consistent with pure insertion.

B and C Reactions (Fig 8.4) The B and C RT’s should differ but they do not.

A and C Reactions (Fig 8.5) Now RT’s differ as they should but force is not consistent with pure insertion.

Confounding Stimulus Intensity Confound – when one or more independent variable is simultaneously varied so we can’t tell which is responsible for an effect. In Ulrich et al.’s experiment (green LED to left or right), two things were varied: Mapping of stimulus to response (one hand vs two) Apparent brightness (focusing on a single light instead of both lights) Replaced by letters X and S not lights.

The Same Experiment using Letters not LED Lights (Fig 8.6) Now RT differs as it should and Force supports pure insertion.

Results using Letters When X vs S is used, the confound of stimulus intensity is eliminated (controlled) and the results support pure insertion. Part of the problem is that all three Donders experiments were not presented: Authors wished to avoid transfer effects that occur in within-subject experiments. To avoid this, present all three Donders conditions or do the experiment between-subject.

Speed Accuracy Tradeoffs RT cannot be used as the only dependent variable because subjects change accuracy to maximize speed: Speed & accuracy are sometimes inversely related. RT & Accuracy must be jointly considered. Examining more than one dependent variable may be crucial to understanding the processes involved in a task.

Theios (1973) Subjects had to name a digit presented visually. Probability of the digit was varied from 0.2 to 0.8. Reaction time was unaffected by probability of the digit, but accuracy was greatly affected. Highest error rate with lowest probability. To increase accuracy (keep error rate constant), RTs would have to increase.

RT and Error Rate as a Function of Stimulus Probability RT stays constant but as stimulus probability increases, errors decrease.

Dual Task Processing

Speed-Accuracy Tradeoff SOA (S1-S2 Interval in ms)

A Central Bottleneck We can only process one thing at a time within a single modality (vision, hearing). Central cognition may be the most important bottleneck – the central bottleneck. Whether two tasks can be done at once depends on whether they compete for the same resources. Schumacher dual-task experiment.

Stimulus Onset Asynchrony (SOA) Pashler presented a modified Donders B task in which S1 and S2 were not presented simultaneously. The interval between them is called stimulus onset asynchrony (SOA) The shorter the SOA, the greater the effect on RT and errors. The period where one task interferes with the other is called psychological refractory period.

Dual Task Processing

Pashler’s Paradigm Task 1 – hear a tone and press a key with the left hand. Task 2 – vocally call out the name of the highest digit in a display of eight digits. When subjects are not required to respond quickly to Task 2, accuracy is not affected. It only occurs with a requirement to make a speedy response.

Capacity Sharing Explanation Response selection S1 Response selection S2 The resource is shared during the time when the tasks overlap.

Explanations Central bottleneck models – some common internal processing stage is required by both tasks, creating a bottleneck for resources. Central capacity sharing models – a resource called capacity must be split across the two tasks, reducing the capacity available to either task and reducing efficiency. Both models predict the same observed results – the author prefers capacity models.

Stress and Cognitive Control Two possible effects of stress on cognition: Stress requires attention (capacity) thus decreasing performance. People adapt to stress by finding more efficient ways of doing tasks increasing performance (strategies change). Steinhauser’s experiment: Long/short interval cues to respond to digit or letter – 6M Is letter a consonant/vowel, is digit odd/even?

Results For the low stress condition there was an interaction between stimulus interval and task repetition (same task next or different next). No interaction in the high stress condition. Under low stress there was a relatively higher cost to changing tasks quickly. Under high stress the cost was the same. This is consistent with the idea that under stress cognitive strategies change.

Change Blindness Demo http://viscog.beckman.illinois.edu/flashmovie/23.php