Australasian Science — May-June 2017

sion flawed, as the only choice participants were free to make

was the timing of a single action.

Recently another team performed a similar experiment, but

here participants had a choice to use their left or right hand to

push a button. These scientists used fMRI to drum in on neural

activity in the parietal cortex, the area of the brain responsible for

processes prior to the final movement command. They claimed

they were able to predict the side chosen at above chance levels

as early as 7 seconds before participants were aware of their deci-

sion (http://tinyurl.com/zpb4s4d). This is a controversial view-

point at odds with our everyday experience that we have complete

agency over what we do and when we do it.

Although we don’t ever encounter anything to suggest that

free will is an illusion, its properties are malleable. In another

experimental paradigm, participants click a mouse and a light

flashes on the screen in front of them (http://tinyurl.com/

hywuere). There is a short delay between the click and the flash.

The task is repeated many times to strengthen the expectation

that clicking the mouse causes the flash. In the test trials the delay

is dramatically reduced without the participants’ knowledge.

Remarkably, participants now judge the light as occurring

before the click.

Having participated in a similar experiment myself, I still felt

that my click caused the light to flash, even though I perceived

the flash preceding the click. This paradox is testament to the

strength of the illusion of free will, that even when we are presented

with sensory input shouting determinism we choose free will.

Time Creates Space?

Adaptation paradigms are also well-suited to investigating how

our brain maps space, in particular the skin. These maps of our

body form enable us to make accurate movements. When adja-

cent locations on the skin are stimulated at around the same

time, they come to be represented adjacently in the brain. The

more times this coincident stimulation occurs, the stronger the

neural connections become. Thus, to map space, we rely on

assumptions about the statistical properties of stimuli hitting

the skin. Part of the reason this occurs is that we have poor spatial

resolution on the skin but excellent temporal resolution.

Our own experiments have probed further to elucidate the

mechanisms of how motion, rather than discrete touch, influ-

ences the mapping of space by the brain. We exploited the

tunnel effect: when a moving object passes behind something

and returns out the other side quicker than expected it is still

perceived as a single object and not some other object. However,

the “object constancy” gained from the tunnel effect comes at

the cost of influencing the brain’s map of space.

When we used a similar setup on the skin (Fig. 1), partici-

pants felt that the untouched skin patch covered by a band

(equivalent to the tunnel) had shrunk. Participants had felt

the brush along the entire length of their forearm, but their

forearm felt shorter.

Our constant velocity model showed that the speed before

and after the band can predict the distance of the missed patch

sensed by participants. That is, the brain uses the known param-

eters – the crossing time and the speed – to calculate the

unknown distance of the missed skin patch (velocity × time),

thus influencing their sense of spatial resolution on the skin.

A similar mechanism is responsible for the failure to see your

eyes move when you shift your gaze from one eye to the other

in a mirror. The sensory information hitting your retina during

this eye movement (a saccade) is suppressed due to its low

quality, so what you perceive during the saccade is actually an

estimate using information from each end of the saccade.

Although this mechanism distorts space–time, it stabilises

our visual field, which is important given we make tens of thou-

sands of saccades every day. This bias is also why, when you

first gaze at a clock, the second hand can appear eerily frozen

for a moment.

Many Clocks

Until recently a clock model of time perception dominated

the field. The main concept of this model was that there was one

clock in the brain. It had a counter, an accumulator and a switch.

The counter generated pulses at regular intervals, which were

26 ||MAY/JUNE 2017

Figure 1. A soft paintbrush is moved back-and-forth along the

forearm. It moves at 15 cm/s, but in the middle 10 cm it moves at

100 cm/s. The brush is not felt in this 10 cm space as a band

occludes the brush from skin contact. The sensation of the brush

on the middle skin patch is perceptually “filled in” from the

traversing motion. Participants commented that they felt the

brush in a continuous motion along their arm, and that their arm

felt shorter.