summed by the accumulator. The switch reset the accumulator
when necessary.
This model seemed to account for perceptual illusions such
as the oddball effect (Fig. 2). In this illusion, the same object such
as a pineapple is flickered on a screen, but unexpectedly the
pineapple is replaced with a novel object like a pool ball. When
participants judge which stimulus in the stream was displayed
for the longest, they consistently pick the pool ball, as if time
has expanded.
Proponents of the clock model account for this by purporting
that the oddball stimulus increases arousal, leading to an increase
in the counter rate, with more ticks equating to more time.
However, others found that the neural response to the repeated
object in the oddball effect may adapt or diminish, a
phenomenon known as repetition suppression. Thus the oddball
only seems to appear for longer because the repeated object is
perceived to appear for less time.
If the single clock model is incorrect then how do we tell time?
A/Prof Derek Arnold of The University of Queensland was part
of a team that made a key discovery favouring a multi-clock model
(http://tinyurl.com/zr4h3yl). In the experiments, participants
viewed a small flickering stimulus for 15 seconds to adapt the eye.
After adaptation, the perceived duration of a 600 ms stimulus
displayed in the same visual region declined by as much as 20%.
If the test stimulus was placed at another location its perceived dura-
tion was unaffected as that location remained unadapted.
This is most consistent with a new model of time in which
independent clocks track the duration of the different images.
This new model requires a shift in our thinking, as it is suggests
that we have no mental representation of time and contrasts with
our other senses, which have well characterised representations
such as the body map for the sense of touch.
The emerging view is that neural networks serve these clocks.
These networks are analogous to how the ripples in a pond can
be used to establish when a pebble was dropped in.
In 2016, a team from UCLA tested if brain slices from mice
could learn to tell time (http://tinyurl.com/zfz53a2). The cells
were persistently stimulated with bursts of light that lasted
100–500 ms, and the cell’s electrical response was measured.
Would cells trained with one stimulus interval show an
enhanced response to that interval over other intervals? The
answer was in the affirmative: cells in a dish could self-organise
into time-keeping circuits and recognise the correct interval.
This in vitro finding paired with the perceptual findings fits
well with a non-clock model, but suggests that it is nurture
over nature when it comes to learning time.Turning Back the Clock
Our processing of time and time sense degrade as we age. The
elderly are less able to correctly detect the order of sounds, and
hence miniscule changes in the timing of syllables that help us
distinguish different words may be lost to them. Can these
deficits be fixed? Can an everyday person improve their sense
of time?
In the late 19th century, psychologist William James wrote:
“Like other senses, too, our sense of time is sharpened by prac-
tice”. At the time little was known about the veracity of this state-
ment, as the only known data came from the
self-experimentation of Max Mehner.
A century later, prominent neuroscientist Michael Merzenich
tested these claims (http://tinyurl.com/zrq5vjt). Participants
in the experiment completed almost 1000 trials per day for 10
days. On each trial, two tones were played, separated by a small
interval. Participants had to select which tone they felt was
longer. After 10 days of training, the difference in duration
between the tones required for participants to correctly discrim-
inate their duration had halved. This was great news as it suggests
that some types of hearing loss could be reversed. Further exper-
iments confirmed that, in the right conditions, the learning
generalises to other tones and intervals.
Today’s brain training apps make similar claims, but whether
they work and have therapeutic potential is a question for time.
Jack Brooks is a PhD student investigating touch perception and body representations with
Neuroscience Research Australia at The University of NSW.MAY/JUNE 2017 | | 27Figure 2. The oddball effect. Participants judge the duration of an image flickered on the
screen. Its perceived duration (left axis) diminishes every time it is displayed. If replaced
with a novel object like an eight-ball, the perceived duration returns to the baseline – the
oddball effect. The perceived duration is mirrored by neural responses (right axis).