From: diwadkva@ctrvx1.vanderbilt.edu (Vaibhav A. Diwadkar)
Newsgroups: rec.sport.cricket
Subject: On playing swing bowling ......
Date: 14 Aug 92 01:41:00 GMT
Organization: Vanderbilt University
News-Software: VAX/VMS VNEWS 1.41

Since there have been quite a few informative postings on the physics of
swing/seam bowling, I thought I'd contribute to expanding the issue with
a brief post on what I know about the perceptual aspects of playing 
swing bowling.  I had taken a class in Perception in the past and there
were some readings in it on the role of eye movements in the perception of
action and events.  I dug up some references and I thought I'd report on what
I'd gleaned from all of that.

To begin with, here's a brief background on eye-movements.  There are three
basic kinds, two of which are relevant to this issue.

Saccadic ("c" are pronounced as "k") eye movements are ballistic in nature,
that is they are extremely fast and "jerky" (I think the term "saccade" is
derived from a French word which means "to jerk").  One essentially engages
in such eye-movements when one wants to foviate (focus) on an object which
is at our visual periphery.  Thus anytime you rapidly shift your visual gaze
from one object to another you execute a saccade.  An obvious example is
reading, which involves rapidly shifting focus from one word to the next.
Incidently saccadic movements take place at remarkable velocities (upwards
of 15 cm per second).  I guess you could saccadic movements as "discontinuous"
though I have not seen that term being used in the literature.

Pursuit eye-movements are what you engage in when you are "pursuing" a target
(such as a cricket ball).  Unlike saccades, these movements are smooth.
There are two sources of information which guide such visual pursuit; one is
the "lower level" information about the target (its velocity etc.) and the
other is the "higher level" which involves the perceivers cognitive 
expectations about the yet to unfold motion of the target.  The "trick" for
the observer is to keep the target sharply imaged on the fovea at all times,
that is to maintain a relatively stationary image of the target on the 
retina. This is easily accomplished for slow moving targets.  The problem
occurs when fast moving targets are to be pursued, the eyes cannot follow
the target in such cases (I don't know what the threshold is) and the 
target's image on the retina tends to "slip".  In short dynamic visual
acuity is inversely proportional to the speed of the target.

The question now is how do these two kinds of movements relate to playing
fast bowlers, coping with late swing etc.

The problem is that fast bowlers (some of them atleast) bowl at speeds
which surpass the eye's ability to continually pursue the ball.  Thus
the problem for the batsman is quite acute.  He is expected to "watch 
the ball" at all times when it is physically impossible for him to do
so.  Studies of this phenomenon have been done with baseball batters who
are in a somewhat similar position to batsman in cricket.  Apparently,
experienced baseball batters cannot follow the path of the ball accurately.
Instead, batters execute a "predictive saccade" soon after the ball leaves
the pitcher's hand.  Essentially the batter pursues the ball for a split 
second after it leaves the pitcher's hand and then "calculates" where
he expects the ball to be when it reaches him and rapidly shifts gaze to
the expected vicinity.  Better batters pursue the ball further in its
flight before making "predictive saccades" than poorer batters (thats why
they are better batters :-).  This explanation transposes quite well IMO
to the cricketing domain.

There are two reasons why fast bowlers are so much more effective than 
medium pacers (apart from the "fear" factor :-).

1.  The sheer speed and the time constraints (as noted above) this places
on the visual system.

2.  The speed is bad enough but they generate what is known as "late swing" 
(I am not aware of the physics of "late swing", maybe someone can fill is in on 
that?).

Since the best a batsman can do is pursue the ball for a very brief instant
before generative a predictive saccade, its probably vital that he glean as
much information about the ball's movement in this brief time frame.  After
the decision is made to abandon the pursuit, the predictive saccade is
made largely on the basis of information gleaned from the early half (or less)
of the delivery.  Unfortunately, the early half of the delivery is not all
that informative if the ball swings very late and the result quite often
is a gleeful group of slip fielders :-).  Of course with experience and
rising skill batsmen can factor this in while using other bits of information
(where is the shiny side facing in the bowlers hand, what direction is the
seam tilted in etc.) to aid him in his task.  I imagine that the better 
batsman (like the better batters) also pursue the ball further in its flight,
but essentially playing the fast stuff demands lots of guts and some very 
intelligent "guesswork".  Since spin bowling is a very different cup of tea,
its no surprise that batsman (Allan Lamb comes to mind) can be adept at 
playing one form of bowling but quite lost at playing the other.

The point is that all swing bowling is problematic but swing which begins
earlier in the balls flight is easier to deal with than swing which begins
later.  Hence bowlers like Manoj Prabhakar swing the ball a great deal but
bowlers like Waqar Younis present many more problems to the batsman.

While many of you probably believe that you "watch the ball" all thru its
flight, its unlikely that you do and if you indeed do, you should be 
playing test cricket instead of reading rsc :-).  Personally, I think
my predictive saccade happens far too early in the ball's flight, thus 
explaining my rather paltry batting average :-(.

Regards, - Vaibhav.

BTW, if you are interested in more reading, any standard text in Perception
should have a chapter(s) on the perception and action of events.  The 
reference for the baseball study I alluded to is:

Bahill, A. T., and Laritz, T. (1984).  Why can't batters keep their eyes on
	the ball?  American Scientist, 72, 249-253.
Newsgroups: rec.sport.cricket