Law professor Tey Tsun Hang was able to recount in court over the past two days, "vivid" details of his ordeal at the hands of anti-graft officers when he was being questioned at the Corrupt Practices Investigation Bureau (CPIB) headquarters last year.

But he had made no mention of such information when any of the CPIB officers took the stand in January during the beginning of the sex-for-grades corruption trial.

This was the recurrent argument made by Deputy Public Prosecutor Andre Jhumabhoy on Wednesday, during his cross examination of Tey, 41, who faces six charges of corruptly exchanging gifts and sex for grades.

The DPP told the court that Tey had essentially made two differing accounts before the court.

"One would be forgiven to see you were simply making this up as you went along," Mr Jhumabhoy put to Tey during proceedings. "It is only here and now when he is not here, not able to give you a response, that you are happy to say he slammed this, he swore like this, he pointed this."

Tey, however, disagreed and said when he conducted his own cross-examination in January that he was in a "fragile" state and added that it was also the reason why his solicitor had applied to recall the CPIB officers to the stand.

"You don't expect someone under psychoactive drugs for some time to be as good as Tan Chee Meng or Davinder Singh," he said, referring to the two prominent senior counsel who are not involved in the case. "I (now) have better recollection," added Tey. "Literally, my recollection is getting better these few months."

The trial within a trial that is ongoing will determine whether statements Tey made to the CPIB are considered voluntary and can be admissible in court.