Attention, decision-making and Fitts Law

Attention picks one stream and cannot track them all

Intuition

At a loud party you can follow one conversation and still catch your own name from across the room. Attention is a filter, and it works within limits.

Attention is selecting what to concentrate on at a point in time from the mass of stimuli around us. It lets us focus on information relevant to what we are doing, and it limits our ability to track every event. How easy it is depends on our goals and on how information is presented. The two hotel-room price activities on the slides (find a double room price in two differently laid out listings) are exercises in how information presentation changes how easily you find data.

Two forms appear in the lecture.

Compare

Selective attention

Awareness of sensory information. Linked to Broadbent’s early selection model.

Divided attention

Consciously dividing mental resources between tasks at once, which is multi-tasking. Linked to Kahneman’s capacity allocation model.

The Stroop effect demonstration asks what it tells you about how we process information: reading is automatic and interferes when the task is to name the ink colour. The lecture leaves the interpretation as a question, so treat this as the point of the exercise rather than a quoted result.

Pitfall

Do not swap the two names. Broadbent is the early selection model (selective attention). Kahneman is the capacity allocation model (divided attention). Worksheet W3 asks you to explain the difference and to say whether we are really multitasking.

Reflective cognition needs help and saved preferences

Reasoning, decision-making and problem-solving are reflective cognition: thinking about what to do, the available options and their consequences. The lecture’s design implications are to provide information and help pages that are easy to access for people who want to understand how to do something, and to let users set or save their own criteria or preferences.

Fitts Law predicts how long a pointing movement takes

Time to respond to a stimulus is reaction time plus movement time, and movement time depends on age, fitness and similar factors. Fitts Law predicts the time needed for a targeted motor movement. The time to select a target depends on the distance to it and its width, plus device-specific constants.

The slide formula did not extract cleanly. The standard form consistent with the slide variables is:

$T = a + b \log_2\left(\frac{D}{W} + 1\right)$

where $T$ is movement time, $D$ is the distance between pointer and target, $W$ is the width (size) of the target, $a$ is the time to start and stop moving (device specific) and $b$ is the inherent speed of the device (device specific).

Exam detail

Larger W lowers time and larger D raises time. a and b depend on the device. Design consequence: make frequent targets big and near the pointer. The summary slide says our hand-eye coordination follows laws such as Fitts Law.

Try it: worked example

Illustration only: a button is 200 px away. Compare 20 px wide with 40 px wide using the log term.

Show answer and steps

AnswerThe log term falls from about 3.46 to about 2.58, so the wider button is faster to hit.

1. These numbers are chosen to show the trend. They are not from the lecture.
2. For W = 20: D/W = 200/20 = 10, so D/W + 1 = 11 and log2(11) is about 3.46.
3. For W = 40: D/W = 200/40 = 5, so D/W + 1 = 6 and log2(6) is about 2.58.
4. With the same a and b, the smaller log term gives a shorter time T. Doubling the width helps, but not by half.
5. Worksheet W3 links an interactive Fitts Law demo so you can see the same trend.

Recall

Write Fitts Law in words and name the two things you can change in a design to speed pointing.

Time grows with the distance to the target and shrinks with its width. Make targets larger and closer.